Abstract: High-quality diamond films are prepared using CH₄/H₂ gas by the microwave plasma chemical vapor deposition (MPCVD) method. Under 150 W low microwave power, the influences of the substrate pretreatment method, deposition pressure, and volume ratio for preparing high-quality diamond films are studied. The results show that the high volume ratio is not conducive to the size control of diamond particles, and the presence of secondary nucleation can obtain diamond films with near nanometer particle size; the larger deposition pressure is conducive to the preparation of dense and uniform diamond films. The impact of substrate pretreatment for the film deposition is obvious, and the ultrasonic treatment with a methanol suspension containing diamond powder is the most effective seeding method.