Abstract: Piston-crank mechanism for internal combustion engines, which comprises a piston, a crank-shaft having a crank-pin, a connecting rod for connecting the piston with said crank pin, an externally toothed gear disposed in side-by-side relationship with respect to the connecting rod and supported for rotation on a pin member extending axially from the crank pin, an internally toothed stationary gear having a member of teeth twice that of the external gear and meshing with the external gear, a pair of rotating balance weights provided on the external gear at the opposite sides thereof in such a manner that they will be in the lower position when the crank pin is in the top dead center, and revolving balance weight means provided on the crank shaft on the side opposite to the crank pin.