Group Science Learning (GSL) is collaborative problem solving based learning by design to improve collaborative problem solving skills, science process skills, and self-confidence of primary schools teacher candidates. This research aims to analyze the effectiveness of GSL model to improve collaborative problem solving skills and self-confidence of primary schools teacher candidates on basic science learning in academic year 2018. This research used one group pre-test and post-test design on 119 primary schools' teacher candidates that were divided into four groups. The data collection methods were conducted by using collaborative problem solving skills test that was emphasized on participation indicator, perspective taking, social regulation, learning and knowledge building, and task regulation; questionnaire of self-confidence was emphasized on: ability indicator, motivation, and perseverance. The data analysis was done by using Paired sample t-test and n-gain. The results showed that there was a significant increase in collaborative problem solving skills and self-confidence at [alpha]=5%, with post-test and n-gain score average in high category for all four groups. Thus, the GSL model is effective to enhance collaborative problem solving skills and self-confidence of primary schools' teacher candidates.