The aim of this thesis is to study fuel changing from LPG to NG in copper melting system at Bangkok Cable Co., Ltd. The copper melting furnace is modified in order to use NG as fuel instead of LPG. In addition, Performance of copper melting furnace will be improved. When LPG is used as fuel for the furnace, the gas temperature is 364.5 degree celcius. Heat loss from exhaust gas is about 48.61%. And thermal efficiency of copper melting furnace is approximately 41.34%. When NG is used, the flue gas temperature is about 406.3 celcius ; heat loss from exhaust gas is 40.25%, and thermal efficiency of copper melting furnace is approximately 45.52%. To summarize, when NG is used instead of LPG furnace thermal efficiency is improved. The accumulated oxygen in melting copper is kept lower than 150-250 ppm, the requirement for good copper quality. The company can save the fuel expense of nearly a million baht per year (56 baht per ton).