To provide a rear part vehicle body structure of a motorcycle for facilitating connection of a harness or the like to a battery, and hiding a driver's seat mounting part to enhance appearance.

A fuel tank is mounted on a pair of right and left tank rails. While a pair of right and left seat rails 12 extending on rear end parts of the tank rails, a driver's seat 14 is provided on the seat rails 12, and the seat rails 12 are formed wider in a vertical direction in a side view. While an upper edge thereof is mounted on the rear end parts of the tank rails in a frontward lowered inclined state, a front lower part of the driver's seat 14 is mounted on a seat mounting bracket 33 formed so as to protrude above upper edges of the seat rails 12. Substantially triangular clearance 36 in the side view through which an opposite side can be seen from one side is formed between a bottom plate 29 of a driver's seat 14 bottom surface and the upper edges of the seat rails 12. The battery 37 with an upper surface provided with a connection terminal 39 is provided in a position where the connection terminal 39 faces the battery from the substantially triangular clearance 36.