In this paper, we present a method which uses the trajectories of an object moving on a physical plane and simultaneously calibrates multiple cameras. This method can be useful for stereo visions and an intelligent multi-camera monitoring system. It is assumed that the extrinsic parameters of one camera are calibrated and the intrinsic parameters of each camera are known. Two approaches are introduced. One uses the projection among the image coordinates of the cameras, and the other handles the relation among cameras’ image coordinates and the world coordinate. The results obtained by these two approaches are compared with the result presented by the standard method and their utility is examined.