We investigate the effects of strain and film thickness on the coercivity of La0.67Ca0.33MnO3 films grown on SrTiO3 and LaAlO3 substractes. Textured microstructure with the grain [0001] pseudo-cubic axis normal to the film surface is obtained. The average grain size decreased with decreasing film thickness t. The magnetic anisotropy depends upon the substrate-induced strain. Firstly, the intrinsic coercivity, HC increases as t varies form 5 nm to 10 nm, then attains a maximum at t～10-25nm. A further increase in t yields a gradual reduction of iHC, By the analysis of the dependence of iHC on the measured direction of magnetic field, the coercivity mechanism is assumed to be mainly determined by the nucleation of reversed domain and the domain wall pining for t≤nm and t≥25nm, respectively, which were further verified by the investigation of the initial magnetization curves for the LCMO/STO films with t=5,10,25 and 400nm, and the minor magnetic loops for the LCMO/LAO films with t=5 and 50nm. Moreover, the size of the inhomogeneous region in which the nucleation or pinning takes place is discussed.