A lower part structure of a motorcycle is provided which enables air having flowed past a radiator to be guided to transversely outer sides more efficiently. In a motorcycle, a radiator is disposed forwardly of an engine and rearwardly of a front wheel, and an air guide port opening forward is provided forwardly of the radiator. A body frame and the radiator are covered with a cowling. The cowling includes side cowls covering lateral sides of the radiator, and an undercover covering a lower portion of the body frame rearwardly of the radiator. The undercover includes a V-shaped part, and air guide surfaces extending obliquely from the V-shaped part toward transversely outer rear sides.