Natural gas and liquefied petroleum gas are the most important alternative fuels for gasoline in spark ignition engines, for many reasons, such as large world reserve of these gasses, high heating value, high octane number, low emissions emitted from burning them in engines, and their small prices compared with gasoline. The practical study conducted with these two fuels to operate single cylinder with variable compression ratio, speed and spark timing Ricardo E6/US, and its performance was compared with that resulted from running the engine with gasoline. The results appeared that the HUCR (higher useful compression ratio) for gasoline was 8:1, 10.5:1 for LPG and 13:1 for natural gas. Results appeared that spark timing was advanced when using NG more than other used fuels, because of its low flame speed propagation. The study conducted that brake power of LPG and NG were less than that for gasoline at CR=8:1, but they became closer when the engine operated at HUCR for each fuel. The results showed that specific fuel consumption for NG less than that for LPG, which was less than that for gasoline on a mass basis. Also, the exhaust gas temperature for NG was found to be less than LPG, and it was for LPG less than that for gasoline.