This quasi experimental research aimed to find out the effectiveness of problem based learning model in terms of students mathematical communications skill. This research used posttest only control group design. The population of this research was all eighth grade students in SMP Al-Kautsar Bandar Lampung in academic year of 2016/2017 which were distributed into 8 classes. The samples of this research were students of VIII-C and VIII-D classes which were chosen by purposive sampling. The research data were obtained by students mathematical communications skill test. Analysis data of this research used t-test and proportion test. Based on the result and discussion of this research, it was concluded that the problem based learning model was effective in terms of students mathematical communications skill.Penelitian eksperimen semu ini bertujuan untuk mengetahui efektivitas model pembelajaran berbasis masalah ditinjau dari kemampuan komunikasi matematis siswa. Penelitian ini menggunakan desain posttest only control group design. Populasi penelitian ini adalah seluruh siswa kelas VIII SMP Al-Kautsar Bandar Lampung tahun pelajaran 2016/2017 yang terdistribusi dalam 8 kelas. Sampel penelitian ini adalah siswa kelas VIII-C dan VIII-D yang dipilih dengan teknik purposive sampling. Data penelitian diperoleh melalui tes kemampuan komunikasi matematis siswa. Analisis data penelitian ini menggunakan uji-t dan uji proporsi. Berdasarkan hasil dan pembahasan penelitian, diperoleh kesimpulan bahwa model pembelajaran berbasis masalah efektif ditinjau dari kemampuan komunikasi matematis siswa.Kata kunci: efektivitas, komunikasi matematis, pembelajaran berbasis masalah.