We described the growth of undoped ZnO thin films and their optical properties changing with a various growth temperature. The undoped ZnO thin films were grown on $c-Al_2O_3$ substrates using pulsed laser deposition (PLD) at room temperature, 200, 400, and $600^{circ}C$, respectively. Field emission microscopy (FE-SEM) measurements showed that the grain size of undoped ZnO thin films are increasing as a increase of growth temperature. In addition, we were investigated that the structural and optical properties of undoped ZnO thin films by x-ray diffraction (XRD) and photoluminescence (PL) studied. Also, we could confirmed that the exciton luminescence was strongly related to charge trap by grain boundary of the samples using micro-PL measurement.