The present paper describes the mechanical losses of a swing-piston type rotary compressor, which has a groove on the outside surface of its piston and a vane engaging with the groove at the tip, compered with a rolling-piston type rotary compressor. Theoretical analysis showed an improvement in mechanical efficiency, attributable to reduction of mechanical loss from the reduced sliding velocity at the vane tip. Experimental results showed that the swing-piston type rotary compressor demonstrated higher mechanical, indicated, volumetric, and compressor efficiency than the rolling piston type one. Due to the increased sliding surface area and the reduced sliding velocity between the vane tip and the piston of the swing piston type, efficiency was improved and reliability may be improved.