Abstract: The effect of argon concentration on the grain size of diamond film at different pressures was studied by hot fibre chemical vapor deposition (CVD) device, and the critical value of the size transition at different pressures was analyzed. The diamond films were analyzed by SEM and Raman spectroscopy. Results show that with the increase of argon volume fraction, the grain size of diamond film will be smaller and smaller, but the argon volume fraction required for the conversion of diamond film from micron to nanometer varies at different pressures. As the pressure decreases, the concentration of argon scale required for the transition decreases gradually. In the range of 2 kPa to 500 Pa, the argon proportion required for the transition is progressively reduced. The volume fraction of Ar decreases from 60.0% at 2.0 kPa to 52.5% at 1.5 kPa, then 32.5% at 1.0 kPa and finally 20.0% at 0.5 kPa. The quality and internal stress of the diamond films obtained at the critical points are also different. With the decrease of air pressure, the quality of the film increases and so does the internal stress.