Pb0.72La0.28TiO.3(PLT28) thin films have been prepared on Pt/Ti/SiO2/Si substrates by pulsed laser deposition under various oxygen pressu re. Experimental study indicated that the oxygen pressure exerts a strong impact on the microstructure and the dielectric properties of the thin films. The film deposited under an oxygen pressure of 2Pa had a larger dielectric constant an d kept a low dielectric loss. At 10kHz frequency, the dielectric constant was approximately 852 and the dielectric loss was 0.0110 Meanwhile, we found that other La- modified PbTiO3 films have the same relation between dielectric con stant and pressure as the above. Possible explanation is given for this.