PURPOSE: To provide a side cover mounting structure of a motorcycle and a motor tricycle where a pillion rider handlebar and a side cover are surely and closely fixed to each other, and the appearance is improved.

CONSTITUTION: Mounting seats 16, 20 which are screwed down to mounting seats 8, 17 of rear frames 2, 2 extending to the rear part of a vehicle body and a side cover mounting seat 22 where a side cover 23 is screwed down are provided on the lower part of a pillion rider handlebar 13. The mounting seats 16, 20 of the pillion rider handlebar 13 and the mounting seats 8, 17 of the rear frames 2, 2 are screwed down, and at the same time, the side cover 23 is screwed down to the side cover mounting seat 22 of the pillion rider handlebar 13.