A lawn mower having a driver's section and a grass cutting unit forwardly of a vehicle body; an engine mounted between front and rear wheels; and a pair of main frames extending longitudinally of the vehicle body, each of the main frames including a front frame member, and a rear frame member connected to the front frame member and disposed at a lower level than the front frame member. The engine is mounted below the main frames between the front and rear wheels, and has a radiator disposed rearwardly of the engine and having an upper portion projecting upwardly of the rear frame members. The engine is surrounded by a first cover assembly including side covers depending respectively from the main frames, lower covers connecting the respective side covers to lower positions of the engine, and a front cover for closing an area forwardly of the engine. The first cover assembly defines first ambient air intake openings. A second cover is disposed rearwardly of the driver's section for covering the upper portion of the radiator. The second cover defines a second ambient air intake opening in a forward portion thereof. Cooling air flows produced by a fan disposed adjacent the radiator are drawn through the first and second ambient air intake openings, at least a large part of the cooling air flows being exhausted rearwardly through the radiator.