A rotor assembly is mounted in each of an intake-compression chamber and an exhaust-power chamber of a rotary internal combustion engine. The rotor assembly includes a split rotor main body and at least three split blades, which all include a left and a right half. Each blade is turnably assembled to the rotor main body via a pivot shaft. The left and right half bodies are correspondingly provided on their joining faces with at least one recess each, and an elastic element is received in between two facing recesses to normally generate an outward push against the two half bodies while a sufficient looseness is always maintained between the rotor assembly and the cylinder wall to allow smooth rotation of the rotor assembly. Front tips and curved backs of the blades have rollers mounted thereto, so that the blades are in rotary contact with the cylinder wall with reduced friction.