A small riding lawn mower having a grass cutting unit suspended from a body frame between a front axle and a rear axle, an engine supported by a body frame in a position forwardly of the rear axle with respect to a longitudinal direction of the body frame, a hydrostatic transmission disposed rearwardly of the rear axle, and a driver's seat disposed forwardly of the engine. The body frame includes profile members disposed at opposite sides and extending longitudinally of the mower, and a cross plate interconnecting the profile members. A belt transmission mechanism connecting the hydrostatic transmission to an engine output shaft is surrounded by a plate frame structure formed below the body frame. The plate frame structure defines a further independent chamber for accommodating a muffler of the engine. This chamber has peripheral walls shaped to guide cooling air from the engine to the muffler and laterally outwardly of the mower.