Effects of Ar ion damage prior to the phase transformation from pyrochlore to perovskite structure of PZT thin films have been investigated. As the degree of damage increased by increasing the acceleration voltage in the ion mass doping system, the phase transformation temperature decreased such that the temperature could be lowered down to 550$^{circ}C$ when the film was damaged at 15 kV for 5 minutes. When the film was damaged prior to the heat treatment grain size of the perovskite thin films became less than 300${AA}$. It turned out that relatively high value of the remanent polarization (about 30${mu}$C/$ extrm{cm}^2$) as well as improvement of the fatigue characteristics to a large extent is closely related to the fine grain size of thus obtained PZT films.