Abstract: Global positioning system ( GPS) on the accuracy of the clock makes a very high demand. Since the speed and the altitude of the satellite，this paper shows that both effects from the special relativity which make motion clock slow and from the general theory of relativity which makes the clock at higher place in a gravitational field fast be large enough，thus the influence of relativity to accuracy of satellite clocks can not be ignored. This is a very good example when we talk about the application of relativity in college physics course.

Key words: GPS, clock, special relativity, general theory of relativity