Abstract: Evepy second the living creatures on earth are subject to muon irradiation.Here an experiment in the level of undergraduate student majoring in nuclear physics related science is introduced aiming to measure the mean velocity of cosmic muons.The mean velocity of muons at around the sea level is determined as with the time-of-flight method.This experiment can be used for teaching in labs of nlodern physics，nuclear physics or frontier physics.It requires that the students understand the high energy cosmic ray and the basic method for radiation detection.

Key words: muon, time-of-flight method, plastic scintillator detector