Abstract: A method measuring micro-hole diameter is presented in this paper.To perform measurement,a measuring system is developed,which consists of laser sensor head,2D-table,driven by step motors,driven and control circuits based on single chip microcomputer etc.The laser sensor head focus the beam on the measured piece.The reflected light is passed through the optical components before reaching four phase photodiode.The photodiode converts the light into current.The current is different between micro-hole and surface on the piece,and would be processed and controlled through interface and single chip microcomputer.The experiment showed the reliability to obtain the accurate diameter of micro-hole.