Objective　To investigate the effect of formative assessment on skill acquisition of basic life support （BLS） among medical students. Methods　Totally 206 undergraduates were ran-domized into control group （C group） and interventional group （F group）. A BLS lecture was given in both groups. And then， 45 min BLS training and BLS skill assessment （after training） were given in C group. Undergraduates in F group received BLS skill assessment （formative assessment） before training， and 15 min feedback was delivered based on the assessment， then 30 min BLS training was conducted. Skills assessment was conducted again in F group after the training. Student's-t-test was used to compare the difference of skills between the two groups and P<0.05 signifies statistically sig-nificant differences. Results　Score of F group （85.2±7.3） were higher than that in C group （68.2±13.2）， with statistical difference. Conclusion　A formative assessment could significantly improve skill acquisition of BLS among medical students.