A virtual measurement system for geometric surfaces of some specific mechanical components was created with Cage Method and Virtual Probe technique based on Three Model Method. The fundamental constituents of the system, the key part of the Three model Method, measured model, were presented in detail with weighed random number technique. Measuring accuracy may properly be controlled by alteration of the number of sampling points of the measured model. Principle of the measuring system, with extendable function, was put forward and explained. The system was operated by VC++ and OpenGL with characteristics of lifelike, function-extension, and immersion. Virtual measurement on a bar-shaft for cylindricalness is given as an example, and it has been shown that the results have much good agreement with those of experiments and that the system worked stably.