A retractable side frame for flat bed trailer in which a Ushaped frame has side members that are slidably received in channels which in turn are removably mounted adjacent to the sides of the flat bed. The U-shaped frame has telescopic legs which may be extended to contact the ground when it is desired to retract the side U-shaped side frame and thus expose the flat bed so that items can be loaded onto or removed from the flat bed along the sides thereof. A modified form of the device discloses mechanical means for retracting the side frame and another modified form shows that the U-shaped retractable side frame as well as the two channels that receive the side members of the rectractable frame can be formed in sections so that the device can be readily disassembled and stored when it is desired to use the flat bed trailer without the retractable side frame.