Diamond films were synthesized using an oxyacetylene flame on a molybdenum substrate mounted on a water-cooled copper block. The morphology and quality of the synthetic diamond films produced depend on several factors including the substrate temperature, gas flow rates and their ratio, and substrate position. The substrate temperature profile was shown to be influenced by the coolant flow rate, nozzle size, substrate position on the heat sink, substrate position in the combustion flame, and the total flow rate of mixed gas (acetylene and oxygen). In this paper, the effects of substrate temperature and nozzle size on the morphology and quality of diamond films are discussed. Fine-gage thermocouples were used to measure the temperature distribution in the substrate during the process. Scanning electron microscopy of the produced diamond films is used to show the variation of crystal morphology.