A combustion-flame method is used to synthesize diamond crystals in various forms under ambient atmosphere. The growth rate is about 500 mu m/h but this technique presents difficulties to obtain coatings on large area substrates. Graphitization phenomena of the previous deposited diamond film, due to the action of the traversing welding torch and the oxide film developed over the substrate surface outside the previously-coated area are the principal difficulties. The aim of this paper is to present a study of the influence of key parameters on the growth of diamond crystal (growth temperature, oxygen and acetylene flows, distance of the inner flame to the substrate etc.) and to establish an optimum combination of coating parameters for diamond growth on large-area substrate. A carbide tungsten substrate (20 x 20 mm) was completely coated with a diamond film using the linear movement of a welding torch.