A frame unit includes first and second bars spaced apart from each other, third bars disposed between the first and second bars, and connectors removably connecting the third bars with the first and second bars. A connector includes a body fixedly connected with the first or second bar, a casing defining an interior to receive an end of a third bar, and a slot allowing insertion of the end of the third bar into the casing. The interior defined by the casing has narrow and wide sides corresponding to the narrow and wide sides of the end of the third bar. As such, when the end of the third bar is inserted into the casing, the end of the third bar is prevented from any translational movement within the casing. The connectors allow quick and easy connection of bars, and in the meantime enhance the stability of the frame unit.