Penelitian ini dilaksanakan mulai bulan Oktober 2014 di SMP Negeri 5 Tarakan. Jenis penelitian ini adalah eksperimen semu dengan desain penelitian Pretest-Posttest Non Equivalent Control Group Design. Populasi dalam penelitian ini adalah siswa kelas IX dan sampel sebanyak 2 kelas, pengambilan sampel dilakukan dengan teknik cluster random sampling. Analisis data menggunakan bantuan software SPSS 16.0 for windows dengan teknik analisis menggunakan uji Independent sample t-test. Analisis data gain dengan menggunakan uji-t diperoleh hasil menunjukkan thitung (2,004) lebih besar dibandingkan ttabel (1,658). Demikian pula hasil uji-t data observasi kemampuan berpikir kritis siswa menunjukkan bahwa ttabel (15,442) lebih besar dibandingkan thitung (1,658). Berdasarkan hasil analisis data dapat disimpulkan bahwa terdapat pengaruh penerapan model pembelajaran Guided Inquiry (Inkuiri Terbimbing) terhadap kemampuan berpikir kritis siswa kelas IX SMP Negeri 5 Tarakan.

The research started begin October 2014 in SMP Negeri 5 Tarakan. Kind of the research is experimental apparent with research design is pretest-posttest Non Equivalent Control Group Design. Population in this research is IX classes students and the sample of this research as many as 2 classes, taking sampling of this research doing by cluster random sampling technique. The analysis data using software SPSS 16.0 for windows with techniques analysis using test of Independent sample t-test. The data analysis gain using t-test obtained result which show that t-count (2,004) bigger than t-table (1,658). Similarly t-test result the data observation students critical thinking ability show that t-count (15,442) bigger than t-table (1,658). Based on data analysis result can concluded that there are effect of applied of Guided Inquiry learning models to critical thinking ability students of IX classes of SMP Negeri 5 Tarakan.