The effect of the growth pressure and substrate temperature on diamond crystallite size was investigated during deposition by hot filament chemical vapor deposition (CVD). A methane-hydrogen gas mixture was used as the precursor gas. The gas flow rates of methane and hydrogen and the deposition time were kept constant at 4 and 100 sccm and 10 h, respectively. The growth pressure and substrate temperature were varied between 40 and 400 torr and between 1020 and 1250 degreesC, respectively. The structure of the films was characterized by high-resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and micro-Raman spectroscopy. Typically, the diamond crystallite size decreased with increasing pressure and decreasing substrate temperature. For example, with increasing growth pressure at 1100 degreesC, the structure of the film gradually changed from microcrystalline to nanocrystalline diamond and the non-diamond defects increased. As the substrate temperature increased at 200 torr, the structure of the film gradually changed from nanocrystalline to microcrystalline diamond.