Kosakata memiliki kontribusi yang signifikan untuk mendukung keberhasilan siswa dalam belajar bahasa Inggris. Tujuannya adalah untuk menemukan kesulitan dominan siswa dalam penguasaan kosakata kelas X SMK Negeri 2 Malinau Kota. Peneliti menggunakan metode deskriptif kuantitatif. Subjek penelitian ini adalah siswa kelas X SMK Negeri 2 Malinau Kota tahun pelajaran 2020/2021. Jumlah populasi peneliti adalah 216 siswa. Pengambilan sampel dilakukan dengan teknik purposive sampling dengan jumlah sampel 35 siswa. Peneliti mengumpulkan data dengan menggunakan tes kosakata dan kuesioner. Tes kosakata dan kuesiner digunakan untuk menganalisis kesulitan dominan siswa dalam penguasaan kosakata. Analisis data menggunakan analisis deskriptif untuk menganalisis kesulitan dominan siswa berupa tes kosakata dan persentase kesulitan siswa dalam bentuk angket. Hasilnya menunjukkan. Pertama, dari hasil tes kosakata, 48% siswa mengalami kesulitan dalam tata Bahas, 84% siswa mengalami kesulitan dalam pemaknaan, 86% siswa mengalami kesulitan dalam pengejaan, kemudian 63% siswa mengalami kesulitan dalam pengucapan. Kedua, hasil angket yang diperoleh diperoleh berdasarkan dari aspek grammar, meaning, spelling dan pronunciation. Dengan demikian, dapat disimpulkan bahwa kesulitan siswa yang paling dominan adalah grammar.

The vocabulary has a significant contribution to support the successful students of learning English. The objective was to find out the dominant difficulties of students in vocabulary mastery of the tenth grade of SMK Negeri 2 Malinau Kota. The researcher had used a descriptive quantitative method. The subjects were the tenth-grade students of SMK Negeri 2 Malinau Kota in the 2020/2021 academic year. The total population of the researcher was 216 students. The sample had selected by using a purposive sampling technique with 35 students as the sample. The researcher collected the data by using a vocabulary test and a questionnaire. The vocabulary test and questioner had used to analyze the dominant of difficulties in vocabulary mastery in terms of grammar, meaning, spelling and pronunciation. The data were analyzed using the descriptive analysis to analyze student difficulties in the form of vocabulary tests and the percentage of the student difficulties in the form of a questionnaire. The results of the research showed. First, from the results of the vocabulary test, 48% of students were difficulty in grammar, 84% of students were difficult in meaning, 86% of students were difficulty in spelling, and than 63% of students were difficulty in pronunciation. Second, the questionnaire resulted related to the grammar, meaning, spelling and pronunciation. Thus, it concluded that the most dominant student difficulty was grammar.