A bed cover assembly for a pickup truck may include cover guides coupled to upper ends of both body sides of a pickup truck and having guide means such as a guide rail formed inward of the truck; a bed cover coupled between the cover guides to slide along the guide means; and rotary guide rail assemblies hinged to both body sides of the rear of the truck, wherein when the bed cover moves rearward, the rotary guide rail assemblies rotate upward about respective hinge axes thereof and the bed cover moves along guide means formed at the rotary guide rail assemblies.