A vehicle body rear part structure includes left and right rear frames extending longitudinally of the vehicle body, a plurality of parallel spaced cylindrical fuel tanks disposed horizontally between the left and right rear frames with the axes of the fuel tanks extending normal to the left and right rear frames and disposed below the center of a height of the rear frames, and a cross member disposed between two adjacent ones of the fuel tanks and extending between the left and right rear frames. The cross member reinforces the rear frames to such an extent that fuel tanks of larger capacity can be carried on the vehicle body rear part structure.