A method was used for online measurement in numerical contr ol(NC) machine by using virtual instrument(VI). By touching or colliding with a 3D probe, the NC machine tool system returned the measurement results to VI. Acc ording to the specific geometric position relationship between the probe and the workpiece, the compensation and the calculation of form and position errors wer e implem ented. Thus, more comprehensive quality evaluation parameters were derived with better visualization. This paper also described the function and its reali zation of online virtual NC machine focusing on the software realization and err or reduction.