This study investigates the relationship between the critical thinking skills and the reflective thinking skills toward problem solving and the effects of two variables weekly study hours and social activities on reflective and critical thinking. Positive correlations between reflective thinking and four sub-dimensions of critical thinking, analyticity, systematicity, selfconfidence, inquisitiveness, also between critical thinking skills and all three sub-dimensions of reflective thinking, questioning, reasoning, and evaluation, are found. A regression model is obtained to predict the reflective thinking skills toward problem solving with three dimensions of critical thinking skills; self-confidence, analyticity, inquisitiveness. Weekly study hours have a significant effect on both reflective and critical thinking. As time spending on studying increases until 15 hours in a week, questioning and reasoning dimensions of the reflective thinking and inquisitiveness dimension of the critical thinking are increasing. As time spending on studying increases until 20 hours in a week, only evaluation dimension of the reflective thinking are increasing. Being an active undergraduate in studying affect questioning, reasoning, evaluation skills of reflective thinking, and also inquisitiveness skill of critical thinking while social activities affect not only questioning and evaluation dimensions of reflective thinking, but also truth-seeking, open-mindedness, and systematicity dimensions of critical thinking.