Objective　To investigate the effect of a formative assessment on skill and knowl edge retention of basic life support（BLS） in medical students. Methods　237 undergraduate medical students were randomized into the control group（C group）or the interventional group（F group）. A BLS lecture was followed by a BLS knowledge assessment in both groups. And then， the C group received BLS training. In the F group， a BLS skill assessment was conducted before training and feedback corresponding to students' error in the skill assessment was given. Then BLS training was followed in the F group. BLS skill and knowledge were assessed immediately after training and at follow up. One way ANOVA test was used to compare the difference of theory and skill between the two groups. Results　 Better skill was observed in the F group immediately after training[F0=（85.2±7.3）vs. C0=（68.2±13.2）， P=0.00] and at follow up， compared with C group. And better knowledge was observed in the F group at 3 and 6 month after the initial training. Conclusion　A formative assessment signifi cantly improved skill and knowledge retention of BLS in medical students.