The effects of short diblock copolymers on the phase behavior of long diblock copolymer melts are investigated theoretically. Due to the enthalpy gain, the short chains segregate to the interfaces and change the interfacial properties, behaving like cosurfactants in a surfactant system. It is shown that small amounts of short diblock copolymer chains at the interfaces are able to shift the phase boundaries of the system, thus providing a new method to modify the morphologis of long diblock copolymer melts.