ZnSe epilayers were grown on (0 0 1) GaAs substrate by using metalorganic chemical vapor phase deposition. The growth was carried out by first growing a 30 nm thick ZnSe buffer layer at a lower temperature of 360 degreesC, followed by the main growth at 450 degreesC. Surface morphologies and photoluminescence (PL) of the ZnSe layers with various thicknesses were investigated. We found that the ZnSe buffer layer contains (1) high density of islands about SO nm in size and (2) low density of holes about 100-1500 nm in diameter. These two features develop in different ways during the growth at higher temperature. The places containing small islands become rougher first and then smoother, with an increase in layer thickness. After the thickness exceeds 210 nm, the surface becomes atomically flat. The holes, however, favor the three-dimensional growth mode and develop into growth hillocks. Energy dispersive X-ray spectroscopy showed that the hole is mainly made up of Ga-Se compounds, consistent with the earlier reports that Ga2Se3 compounds are the source for the growth of hillocks. PL study showed that the donors, accepters and the Jeep level emissions associated defects are mainly concentrated at the GaAs/ZnSe interface.