The dependence of piezoelectric properties of PZT thin films on film thickness was investigated using atomic force microscope (AFM) assisted domain imaging technique. PZT thin films were fabricated on Pt(111)/TiOx/SiO2/Si substrates at 375degreesC by radio frequency (RF) magnetron sputtering. As the thickness of PZT thin film increased, the preferred orientation of PZT thin film changed from (110) to (100). The shape of tip vibration amplitude was asymmetric and the piezo-response of PZT thin films increased with increasing film thickness. And the piezo-response had an inverse dependence on the volume density of PZT films. We describe the relationship between microstructure and piezoelectric properties of PZT ultra-thin films.