<![CDATA[This research were intended to describe the practicality, effectivility, and effect size of problem solving model to improve fluency thinking abiltiy on Arrhenius acid base theory. This research used one group pretest postttest design in which the 11th grade of science-5, was choosen as sample based on cluster random sampling . The aspect of practicality was determined by lesson plan study implementation based on teacher observation and conspicuousness of model based on students response. The aspect of effectivi-lity was determined by average n-Gain of students`s concept mastery in exami-niation and student activity during the learning process. The aspect of effect size(r) was determined by mean of pretest and posttest in which r score was calculated by using Cohen`s formula. The result that problem solving model has high criteria of practicality and effectivility and large effect size to improve fluency thinking ability.Penelitian ini bertujuan untuk mendeskripsikan kepraktisan, keefektifan, dan ukuran pengaruh model problem solving dalam meningkatkan kemampuan berpikir lancar siswa pada materi asam basa Arrhenius. Penelitian ini menggunakan one group pretest posttest design dengan menggunakan teknik cluster random sampling diperoleh sampel kelas XI IPA5. Aspek kepraktisan ditentukan dari keterlaksanaan RPP berdasarkan obser-vasi guru dan kemenarikan model berdasarkan respon siswa. Aspek keefektifan ditentukan dari rata-rata n-Gain hasil penguasaan konsep di awal dan akhir pem-belajaran dan aktivitas siswa selama proses pembelajaran. Ukuran pengaruh(r) ditentukan dari rata-rata nilai pretes dan postes dengan nilai r dihitung meng-gunakan rumus cohen. Hasil penelitian menunjukan bahwa model problem solving memiliki kepraktisan dan keefektivan sangat tinggi serta memiliki ukuran pengaruh besar dalam meningkatkan kemampuan berpikir lancar siswa.Kata kunci :kepraktisan, keefektivitasan, problem solving, ukuran pengaruh]]>
