Abstract: At the industrial insite conditions, the small size can't be quickly and accurately measured by the traditional measurement way. Based on the need for small size measurement in industry, a kind of photoelectric measuring apparatus to clearance is designed in this paper. We use the linear array CCD (Charge Coupled Device) as a photoelectric sensor and singlechip computer as a main control unit to do the noncontact and online measurement of clearance between the gear and highfrequency quenching inductor, and get a good measurement result. Therefor, the apparatus can monitor the production status and improve the working efficiency. This project is suitable for the application of low cost and high precision measurement.