An optical actuator includes a base, a carrier, a reflecting optical element, a connecting structure and an actuating component. The reflecting optical element is disposed at a first side of the carrier. The connecting structure has a first connecting portion located at the central region thereof and two second connecting portions located at two opposite sides thereof. The connecting portions connect the base to the second side of the carrier. The actuating component is disposed on the base and drives the carrier rotating along an axial line. The axial line is substantially perpendicular to the connecting structure. A projection system including the optical actuator is also disclosed.