A paper feeding apparatus with a paper size determining means includes a paper tray, paper guide, a belt, a roller set, a sensing mechanism and a control unit. The paper tray has a front surface for supporting paper and a rear surface. The paper guide is movably disposed on the paper tray and for abutting against an edge of the paper in the paper tray. The roller set is rotatably disposed on the rear surface. The belt, connected to the paper guide and wound around the roller set, synchronously moves the paper guide and rotates the roller set. The sensing mechanism disposed on the rear surface senses a to-be-sensed component disposed on the belt or the roller set and thus outputs a sensing signal. The control unit receives the sensing signal and thus determines size of the paper.