An automatic focus system of a projector includes a projecting module, an image catching module, a memory, a micro processor, and a focusing module. The projecting module has an image light source to provide image light, and a projecting lens to project the image light on a screen. The image catching module senses an image on the screen. The microprocessor calculates the sensed image to have a definition value of the image and save the definition value in the memory. The focusing module may move the projecting lens is moved along an optical axis under a control of microprocessor. The microprocessor compares the definition values to control the focusing module to move the cam to a position where the projector may project a clear image on the screen.