Penelitian ini dilakukan untuk (1) mengidentifikasi dan mendeskripsikan tipe-tipe kesalahan yang dilakukan oleh mahasiswa semester dua dan empat dalam menuliskan paragraph deskriptif, (2) mengidentifikasi dan mendeskripsikan sumber kesalahan yang mempengaruhi mahasiswa untuk melakukan kesalahan ditulisan mereka dan (3) mendeskripsikan persamaan dan perbedaan kesalahan yang dilakukan oleh mahasiswa semester dua dan empat pada tulisan mereka. Subyek penelitian ini adalah 30 mahasiswa pada setiap semester ditahun pembelajaran 2016/2017. Bertujuan untuk mengumpulkan data, peneliti meminta siswa untuk menulis tulisan deskriptif dengan topik yang telah diberikan. Peneliti juga mewawancara mahasiswa terpilih untuk mendapatkan informasi mengenai kemungkinan penyebab kesalahan yang mempengaruhi mereka melakukan kesalahan. Setelah mengumpulkan dan menganalisis data, ada 2492 kesalahan yang ditemukan pada tulisan mahasiswa yang dianalisis dengan menggunakan Surface Strategy Taxonomy. Ditemukan 803 kesalahan pengihilangan, 274 kesalahan penambahan, 1195 kesalahan pembentukan dan 220 kesalahan penempatan. Sementara itu, menurut Comparative Analysis Taxonomy kesalahan-kesalahan tersebut dipengaruhi oleh beberapa faktor seperti kesalahan perkembangan, kesalahan interlingual, keslaahan ambigu dan kesalahan lainnya. Maka ditemukan 219 kesalahan disebabkan oleh perkembangan, 216 kesalahan oleh interlingual, 457 kesalahan oleh ambigu dan 41 kesalahan oleh lainnya. Membahas persamaan, kedua mahasiswa semester tersebut melakukan kesalahan di aspek verb, article, verb tense, conjunction, preposition, pronoun, adverb and plural. Sementara perbedaannya, struktur kalimat mahasiswa semester dua lebih dalam bentuk kalimat bahasa Indonesia, sementara itu mahasiswa semester empat kebingungan menerapkan pengetahuan mereka tentang grammar. Kesimpulannya, kesalahan-kesalahan gramatika terjadi pada tulisan mahasiswa kerena kurangnya pemahaman mahasiwa mengenai grammar, pengaruh struktur bahasa Indonesia dalam pembelajaran dan faktor terbesar yang bersumber dari mahasiswa itu sendiri seperti gaya belajar mahasiswa.

This research was conducted to (1) identify and describe the types of errors made by the second and the fourth semester students in descriptive writing, (2) identify and describe the sources of errors that influenced the students to commit errors in their writings, and (3) describe the similarities and differences errors made by the second and the fourth semester students in their writings. The subject of this research was 30 students of each semester in academic year 2016/2017. In collecting the data, the researcher asked the students to write a descriptive writing by the given topics. Also the researcher interviewed the chosen students to get information about the possible causes influenced them to commit error. The result showed that there were 2463 errors found in the students’ writings, they were analyzed based on the Surface Strategy Taxonomy. There were found 800 errors of omission, 274 errors of addition, 1174 errors of misformation and 215 errors of misordering. Meanwhile, according to Comparative Analysis Taxonomy those errors were influenced by some factors like developmental errors, interlingual errors, ambiguous errors and other errors. Thus, there were found that were 219 errors caused by developmental, 216 errors caused by interlingual, 457 errors caused by ambiguous and 41 errors caused by other. In describing the similarities, one of the similarities was both those semester students did errors in the same aspects such as verb, article, verb tense, conjunction, preposition, pronoun, adverb and plural. While one of the differences was the structure sentence of the second semester students were more in Indonesian pattern, meanwhile the fourth semester students were complicated in applying their knowledge about English grammar. Briefly, the grammatical errors occurred in the students’ writings were caused by the lack comprehension of English grammar, the influence of the native language pattern (Indonesian) in the process of learning target language and the most factor was sourced from the students itself such as students’ learning style.