A vehicle body rear part structure includes a rear side frame. The rear side frame includes a general part and a slope part. The general part extends substantially horizontally in at least one of an upper surface and a lower surface. The slope part extends so as to slope downward from the general part. A curved part is formed at a boundary between the general part and the slope part. The rear side frame includes two components which are a front side member and a rear side member. The curved part is provided on a joint part between the front side member and the rear side member.