Peneliti mengamati suatu bentuk masalah yang terjadi di kelas yaitu rendahnya kemampuan bernalar siswa di SMP X. Tujuan penelitian adalah untuk melihat pengaruh model pembelajaran berbasis masalah terhadap kemampuan bernalar siswa kelas VIII di SMP X yang dilaksanakan pada tanggal 3 Agustus hingga 13 November 2015. Dalam penelitian ini peneliti menggunakan uji normalitas data untuk data pre-test dan post-test. Jika hasil dari pre-test dan post-test berdistribusi normal maka akan digunakan uji paired samples t-test, sedangkan data yang tidak berdistribusi normal akan menggunakan uji wilcoxon. Setelah itu akan digunakan uji N-gain untuk analisis lanjutan. Hasil analisis yang didapatkan dari penelitian ini adalah penerapan model pembelajaran berbasis masalah berpengaruh terhadap kemampuan bernalar siswa kelas VIII. Pada perbandingan post-test 1 sedangkan post-test 2 tidak ada peningkatan yang signifikan dan dari post-test 2 ke post-test 3 ada peningkatan kemampuan bernalar siswa dengan penerapan metode pembelajaran berbasis masalah. / The researcher was observed the problem that occured in the classroom was the students‟ reasoned ability at SMP X. The purposes of this research is to know the effect of the Problem Based Learning of grade VIII students‟ reasoned ability at SMP X that conducted on August until November, 13th 2015. In this research, the researcher used the data normatlity test for pre-test and pot-test data. If the result of pre-test and post-test were normally distributed, then the Paired Samples Test was used, and for the data that not normally distributed would using the Wilcoxon test. The N-gain test would be use afterward for further analysis. The result of this research was the implementation of Problem Based Learning method had effected to the grade VIII students‟ reasoned ability. The comparison between post-test 1 with posttest 2 shown that there was no significant effect and from the post-test 2 with post-test 3 shown that there was a significant effect of the grade VIII students‟ reasoned ability at SMP X through the implementation of problem based learning method.