The heat treatment effects of the undoped ZnO and Al doped ZnO(AZO) transparent conductive films prepared by rf magnetron sputtering were investigated. The variations of the electrical and optical properties with heat treatment temperature and ambient were studied. The resistivity of the undoped ZnO films heat treated in air and Hz plasma for 1 hour increased rapidly above 200 ℃ and 300 ℃, respectively. And that of the ZnO:Al films heat treated in air also increased rapidly above 300 ℃. On the other hand that of the ZnO:Al films heat treated in Hz plasma was constant regardless of heat treatment temperature. The optical transmittance above 550 nm is about 90 % for all thin films regardless of impurity doping, the heat treatment temperature and ambient.