Basic life support (BLS) training is crucial in improvement of cardiopulmonary resuscitation (CPR) outcomes. Many studies have demonstrated improvement of skills after BLS training but the skills significantly decrease over time. Our study aimed to evaluate the retention of knowledge and skills after training following 2010 BLS guidelines in second year medical students at Faculty of Medicine, Thammasat University. Materials and Methods: One hundred and forty-nine second-year medical students were enrolled in the prospective cohort study. Participants were tested for knowledge and skills of BLS prior to training (pre-test), immediately after training (post-test) and six months after training (retention test). Results: The mean scores of pre-test, immediate post-test and retention-test were 8.52 (SD 1.88), 12.12 (SD 1.52) and 10.83 (SD 1.95), respectively. Improvement in knowledge score post-test and retention test were 3.60 (95% CI 3.22,3.99 P<0.001) and 2.31 (95% CI 1.92,2.70 P<0.001) respectively, compared with pre-test score. In post-test, detection skill, activation skill and compression skill were improved 1.67 (95% CI 1.28,2.19 P<0.001), 5.15 (95% CI 3.41,7.77 P<0.001) and 3.88 times (95% CI 2.24,6.71 P<0.001) compared with pretest evaluation. Comparison between retention test and pre-test was improved detection skill 1.72 (95% CI 1.31,2.26 P<0.001), activation skill 4.4 (95% CI 2.93,6.75 P<0.001) and compression skill 2.56 (95% CI 1.44,4.57 P=0.001). Knowledge decreased 1.29 times in retention test compared with post-test (95% CI −1.67,0.92 P<0.001). In retention test, detection skill increased 1.03 times (95% CI 0.81,1.29 P = 0.810), activation skill decreased 0.86 times (95% CI 0.98,1.10 P =0.24) and compression skill decreased 0.66 times (95% CI 0.45,0.98 P=0.04) compared with post-test. Conclusion: Knowledge and skills of BLS significantly improved after training in second year medical students. However, the knowledge decreased at 6 months after training although the BLS skills still remained.