This study aims to determine the effect of Treffinger learning model on students' mathematical problem solving skill of SMP Negeri 4 Sumbang. It is an experimental research with Posttest Only Control Design. The sampling technique of this research is cluster random sampling by taking two classes; class VIII A as experimental class and class VIII B as control class. Result of post-test shows that average score of the students' problem solving skill in experimental class is 67.10 while the average score of the students in control class is 60.06. After conducting hypothesis testing using the Independent-Sample t Test obtained Sig.=0,016/2 = 0,008, , Sig. < 0.05 or H0 is rejected. It indicates that students' mathematical problem-solving skill of the students following Treffinger's learning model is better than the mathematical problem-solving skill of the students following direct learning.