A paper feeding unit and an image forming apparatus having the same capable of maintaining a constant paper feeding force regardless of a size of paper. The image forming apparatus includes a main body, a printing unit provided in the main body to form an image on paper, a paper feeding unit to supply the paper into the main body, and a force adjusting unit. The paper feeding unit includes a paper feeding cassette to load paper therein, a knock-up plate to guide the paper loaded in the paper feeding cassette to a pickup device, an elastic member to elastically bias the knock-up plate toward the pickup device, and guide sides provided at both sides of the knock-up plate. The guide sides are adjusted along a distance therebetween according to a width of the paper. The force adjusting unit adjusts an elastic force of the elastic member when the distance between the guide sides is adjusted according to a size of the paper. Accordingly, since the elastic force of the elastic member supporting the knock-up plate is adjusted through the force adjusting unit which interlocks with the guide sides moving corresponding to the change of the size of the paper loaded on the knock-up plate of the paper feeding cassette, the paper feeding force can be maintained constant all of the time.