There has been few investigations of the relation between scavenging flow of a two-stroke cycle engine and its efficiency. To investigate the relation, the process in which the water in a transparent cylinder was scavenged by injection of a colored salt solution was photographed. By measuring the specific gravity of contents in the cylinder after scavenging, scavenging efficiency was calculated, and the results were compared with theoretical scavenging characteristic curves. These experiments were carried out on a two-dimensional scavenging flow in a model with a fixed piston and on a three-dimensional scavenging flow in a model with a movable piston. It is concluded that, (1) scavenging efficiency varies continuously with delivery ratio, (2) vortex motion of scavenging does not appear within the delivery ratios of engines which are in practical use, and (3) the deterioration of scavenging efficiency is caused mainly by the pure short-circuiting loss.