The instantaneous center of velocity between two components must first be located in order to analyze the velocity of the mechanism using the velocity instantaneous center method. For some simple mechanism, the velocity instantaneous center between two components can not be determined by the relations between components in analy-zing mechanism. In order to solve the kind of problem, this paper puts forward the mechanism extension method to determine some indefinite instantaneous center of velocity. The mechanism extension method is the way that the ini-tial mechanism is extended to the mechanism consisting of more components and it increases three component com-binations being used for theorem of three velocity instantaneous centers. Meanwhile, it is ensured that both the ex-tended mechanism and the initial mechanism are equivalent. Thus, some indefinite instantaneous centers of velocity in the initial mechanism can be located with its extended mechanism. The application of the mechanism extension method is demonstrated with examples and the results show that the new method is supplied for the kinetic synthesis of mechanism.