The invention relates to rotating bodies of a rotary piston engine. Each rotating body, seen from shape, is formed by combining a round plate with an annular circular surface and n pistons distributed uniformly along the circumferential direction of the round plate, wherein n is an even; and two groups of pistons on the two rotating bodies are arranged at intervals and divide a ring-shaped air cylinder into 2n sealed spaces. In terms of structure, each rotating body is formed by combining and assembling a base plate and a piston rack with n pistons distributed uniformly along the circumferential direction of the base plate, wherein the base plate is a plate with an annular circular surface, which is part of the surface of the ring-shaped air cylinder and is provided with n grooves distributed uniformly along the circumferential direction of the base plate; and the structure of the piston rack is formed by combining a ring with a splined hole as a central hole, n pistons distributed uniformly and ring-piston connection parts which can be just embedded in the n grooves on the base plate. The rotating bodies have the advantages of low processing difficulty and high surface processing quality.