The invention discloses a workpiece dimension measuring instrument for a numerical control milling machining center. The dimension measuring instrument comprises a driving device, radial measuring equipment, end face measuring equipment and a controller; the driving device comprises a bottom plate, a radial probe support, an end face probe support, a measuring equipment advancing and retreating cylinder, a linear guide rail, a radial measuring equipment sliding block, an end face measuring equipment sliding block, a stud, a sliding plate, an adjustable bolt and an adjustable sliding block; all the measuring equipment comprises an arrow fastening screw, an arrow, a probe zero calibration nut, a probe and a measuring bar. The dimension measuring instrument is capable of being installed on the numerical control milling machining center, it is not necessary to stop machines between working procedures and on-machine measurement of critical dimension of parts can be achieved, the problem of mass deviation caused by secondary clamping and secondary alignment of workpieces is avoided, the assistant measurement man-hour is reduced, the production efficiency is improved and the dimension measuring instrument is especially suitable for full-automatic and mass production line operation.