The equipment for on-line measurement of the bore size of a workpiece，is described in this paper. The probe is set up by three inductance sensors，assembled with certain angles. The result of analysis and experiment show that the error of assembling angles between the rods of sensors is the main Cause of the measuring error. The measuring precision is better than ±2μm in the certain case. This equipment is availabe to the monitoring system of machine centres for bore size measuring. In addition，it can also be used for the measuring of eccentricity between the centres of measuring tool and workpiece bore. Especially，it is available in the batch process for the purpose of on-line quality inspection