The effects of knock with varied intensity on spark-ignition engine performance and emission characteristics were investigated using a single-cylinder CFR engine operated by several different fuels. The variation of knock under a fixed engine speed was obtained by operating the engine using different octane numbers of the fuel and the variation of fuel's octane number was made as follows: For gasoline, two fuels having different octane ratings were used to obtain three different octane-number fuels, 85.3, 87.1, and 88.9; for gasoline/alcohol blend fuels, the volumetric alcohol contents in the blend were 0, 5, and 10% to obtain octane ratings of 85.3, 85.7 and 86.2, respectively; for natural gas (with over 94.5% methane by volume), small different amounts of alcohol were introduced into the stream of gas to produce octane numbers of 116, 118 and 120. For the same fuel, the knock intensity was stronger at lower engine speed and lower with high octane number. The exhaust gas temperature slightly increased with increase in octane number for all fuels and this trend did not seem to depend on engine operation speed. The emission of unburned hydrocarbon for alcohol and natural gas fuels increased with knock intensity and for gasoline the trend was the opposite; the effect was more obvious with higher engine speed; and its specific emission (mass/hp) was highest with alcohol/gasoline blend and lowest with gasoline. Regarding the emission of NOx, the higher the octane number the lower the amount of its emission. This trend was strongest with natural gas operated engines at higher engine speed and its specific emission appeared to be highest with gasoline operation and lowest with alcohol/gasoline blend. The increase in power output (and decrease in brake specific fuel consumption) obtained by higher octane rating fuel was most significant in gasoline and very small in gasoline/alcohol blend and natural gas fuels. Some plausible interpretations were offered for those observations.