When three objects move to and fro in the horizontal direction， ifthere are differences of velocities among them， they transform from 2-Dmotionto 3-Dmotion. The relatively slower object(s) tends to be the reference of 3-Dorganization. In this case， one object crosses the others， and one of them occludesthe others. Komatsu and Masuda(1998) and Tommasi et al.(1995) suggestedthat occlusion may influence their 3-Dorganization.  Whether moving objects cross each other or not， the fastest object move roundthe others. Its trajectory is arc in 3-Dspace. We usually perceive 3-Dtrajectory.2-Dtrajectory is unusual perceived.  When one moving object passes the one still object， the shape of the still objectinfluences the appearance of the moving object's trajectory. This change is agreeswith Petter's rule.  When the moving object before occlusion is perceived different from it afterocclusion，3-Dorganization of the objects cannot also be perceived.