We propose a widegap semiconductor, MgxZn1-xO, as a barrier layer for ultraviolet light emitting devices based on ZnO. MgxZn1-xO single layer films and double heterostructures of MgxZn1-xO / ZnO /MgxZn1-xO were fabricated by pulsed laser deposition on sapphire (0001) substrates. The: photoluminescence peak of MgxZn1-xO shifted from 3.36 eV (x=0) to 3.87 eV (x=0.25) with increasing x. Epitaxial double heterostructures were successfuly grown as veryfied by X-ray diffraction and secoundary ion mass spectrometry. Clear exciton emission in double heterostructure indicates that this alloy system is promising for the light emitting devices.