Thermal annealing of ZnO layers was done in N-2 or O-2 atmosphere and their effects were studied. Electron carrier density increases according to reevaporation of O from ZnO if annealed in N-2 atmosphere. On the contrary, it decreases from the order of 10(18) to 10(17) cm(-3) and also optical properties are improved when annealed in O-2 atmosphere at lower temperature. In that case, the number of interstitial Zn (Zn-i) and O vacancies (V-O) decrease, probably because the effective incorporation of O atom diminishes those donor levels. The crystallinity also improves with the annealing. The mobilities of the layer increase up to 51 cm(2)/V s as annealing temperature increases, but are lower at lower temperatures. Solving this problem, annealing may become a promising technique for the fabrication of p-type ZnO layers.