A connection device for connecting a bicycle rack to a connection member on a vehicle includes a clamping unit composed of four quarters and a mounting member is mounted onto the four quarters withinhich the connection member is enclosed. A bolt's head is clamped by four respective protrusions on the four quarters and the shank extends through the top of the clamping unit and the mounting member so as to be connected with a knob. A lock is received in the knob and cooperated with the bolt. The mounting member has a T-shaped slot and two lugs extend from the mounting member. The vertical slot is located between the two lugs and a quick release unit is cooperated with the two lugs so as to control the distance between the two lugs. By using the quick release unit, the four quarters can quickly release the connection member.