Abstract: The unidirectional piston internal combustion engine is composed of a stator and a rotor. Annular grooves are cut on the rotor. Each groove is partitioned with a revolving valve gate to form into one "cylinder" between the rotor and the stator. Pistons are inserted into the cylinders and fixed on the rotor while the valve gates are incorporated in the stator. When the rotor turns, the pistons move relatively with the valve gates, which open to let the pistons pass and close again. The valve gates are operated directly by the pistons. Every two adjacent cylinders are made of a group, one for air compressing and one for combustion and expansion. For each group of cylinders, a one-way air passage valve is provided for compressed air to enter into the combustion and expansion cylinder, and is incorporated in the stator. Inlet and outlet ports are made in the stator. Seals for the valve gates, pistons and between grooves are developed to secure high pressure against leakage.