Abstract: The improved system for liquid cooling of a rotor of a rotary piston mechanism applies to those types having two housing walls spaced apart by a trochoidal shaped peripheral wall to define a multi-lobed cavity within which a multi-cornered rotor is eccentrically supported for planetary movement, and in which the rotor has a hub portion and peripheral wall portions defining therebetween cooling spaces which communicate with the area exteriorly of the rotor through an opening adjacent each end of the hub portion. The improved rotor cooling system comprises a cooling liquid discharge nozzle disposed in at least one of the housing walls and connecting the discharge nozzle to a source of pressurized cooling liquid. The discharge nozzle is positioned so as to direct emitted cooling liquid into the adjacent rotor opening and the cooling spaces in the rotor.