II–VI/III–V heterostructures based on wide-energy band-gap II–VI compound semiconductors have been very attractive because of their potential applications in short-wavelength optoelectronic devices [1–5]. Among these II–VI/III–V mixed heterostructures, ZnSe/GaAs heterostructures have been particularly interesting due to their promising applications for the fabrication of laser diodes and light-emitting diodes operating in the blue-green region of the spectrum [6–10]. However, relatively little work has been performed on II–VI/III–V heterostructures in comparison with III–V/III–V heterostructures due to cross-doping problems resulting from interdiffusion or intermixing during growth and fabrication [11]. Since thermal treatment is necessary for the fabrication processes of several kinds of optoelectronic devices utilizing ZnSe/GaAs heterostructures, the role of the thermal annealing processes is very important in achieving high-performance devices [12]. Therefore, studies of the annealing effects on the surface, the structural, and the optical properties play a very important role in enhancing device efficiency. This letter reports the effect of annealing of ptype ZnSe epilayers grown on n-type GaAs (100) substrates by using molecular beam epitaxy (MBE). Atomic force microscopy (AFM), photoluminescence (PL), and transmission electron microscopy (TEM) measurements were performed in order to investigate the surface, the optical, and the structural properties of as-grown and annealed ZnSe/GaAs heterostructures. The samples used in this study were p-type ZnSe epitaxial layers grown on n-type GaAs (100) substrates in a Riber 32 P system by using MBE. Elemental Zn and Se with purities of 99.9999% were used as the source materials, and the flux ratio, Zn/Se, was approximately 0.5 during ZnSe epilayer growth. The p-type doping was carried out using an rf plasma source to excite nitrogen gas at a pressure of 10−8 Torr in the