Thegrain-size-dependence of 90-domain structure in polycrystalline PbTiO3 thin films is systematically studied by using X-ray diffraction (XRD). The PbTiO3 thin films with different average grain sizes were prepared on various substrates by a sol-gel process. The films on NaCl and fused glass are randomly grain-oriented, while those on (111)Pt/Ti/SiO2/Si are highly f100g cubic index grainoriented. It is found from the XRD patterns of the films on NaCl that with decreasing average grain size from 230 to 80nm, the intensities of high h index (h>l) peaks (hkl), such as (100), (110), (200), (201), (210), (211), etc., decrease rapidly and ultimately disappear, whereas another set of peaks. lkh/, including (001), (002), (102), (112), etc., are still intense. This interesting result suggests that at grain size below a certain critical size an increasing number of grainsno longer show 90-domains, which is confirmed by TEM observations. Meanwhile, X-ray evidence of such a grainsize-related absence of 90-domains is also found for PbTiO3 films on Pt (111) and fused-glass substrates. The volume fractions of single-domain grains (without 90-domains) in the films are estimated from their XRD patterns. By combining SEM and TEM investigations, the critical grain size for the formation o 90-domains is further determined to be near 200nm.