A growth model for the low pressure chemical vapor deposition (LPCVD) of polycrystalline ZnO thin films is proposed. This model is based on experimental observations of the surface morphology and crystallographic orientations of the layers at different thicknesses and growth temperatures. It is shown that the films preferred orientation evolves from c-axis to aaxis as the growth temperature is increased from 110 to 220 Cand then goes back to c-axis at 380 C. At the same time, when the film thickness increases, the surface morphology evolves from small rounded grains to large pyramids at a growth temperature of 150 C. The selection of various preferential orientations under different deposition conditions is attributed to growth competition between clusters initially formed with different crystallographic orientations.