A mounting structure for an air-fuel ratio sensor in a motorcycle is provided which eliminates the influence of water gathered in an exhaust pipe upon the air-fuel ratio sensor and improves the mountability of the air-fuel ratio sensor to the exhaust pipe. The air-fuel ratio sensor is mounted on an exhaust pipe connected to an engine in a motorcycle so that the air-fuel ratio sensor extends radially with respect to a longitudinal axis of the exhaust pipe, and is inclined upward by an angle of 10° or more with respect to a horizontal line H passing through the center of the exhaust pipe 32 in its cross section. The air-fuel ratio sensor is pointed toward the central lateral plane of the motorcycle as viewed in front elevation of the motorcycle.