A piston assembly mounting apparatus for an engine may include a robot having two multijoint arms movable in a predetermined range, a taking-out unit mounted at a free end of one of the arms of the robot and clamping and taking out one of a plurality of piston assemblies, and a fitting unit mounted at a free end of another of the arms of the robot, receiving and clamping the one piston assembly taken out of the taking-out unit, and then inserting the one piston assembly into a cylinder of a cylinder block.