This research aimed to determine the influence of Continuous Professional Development (CPD) training offered by Rwanda Quality Basic education for Capital Human Development Project (RQBEHCD) sub-component 1.2 on upper primary teachers' Pedagogical Content Knowledge (PCK) in mathematics and science, and elementary technology (SET) subjects. The study adopted an explanatory sequential design to collect and analyze data. Quantitative data were collected from Mathematics and SET achievement tests while qualitative data were collected from semi-structured interview. Quantitative data were collected from 290 participants including 166 male and 124 female teachers. On the other side, the qualitative data were collected from 20 teachers including 12 males and 8 females selected randomly. Descriptive and inferential statistics (repeated measures Analysis of Variance were used to analyze quantitative data while interpretive and thematic analysis were used to analyze qualitative data. The results found high statistical significance in mean scores of mathematics teachers between pre and post-test, with a p < 0.001, medium learning gain of 0.37, and an effect size of 0.450. However, there were no significant differences found between male and female teachers (p > 0.05). Likewise, SET teacher's results showed a significant difference in mean score between pre and post-test with p < 0.01, learning gain of 0.20, and effect size of 0.432 but no significant difference was found between male and female teachers (p > 0.05). In addition, the qualitative results indicated that the CPD program enhanced engagement, interest, confidence, and self-efficacy in teaching Mathematics and Science. Thus, we recommended that the CPD program should be done nationwide to maintain teachers' PCK in mathematics and science.