In this paper, an experimental study was performed to investigate characteristics of flame propagation and knocking combustion of hydrous (10% water content) and anhydrous ethanol at different air/fuel ratios in comparison to RON95 gasoline. Experiments were conducted in a full bore overhead optical access single cylinder port-fuel injection spark-ignition engine. High speed images of total chemiluminescence and OH* emission was recorded together with the in-cylinder pressure, from which the heat release data were derived. The results show that under the stoichiometric condition anhydrous ethanol and wet ethanol with 10% water (E90W10) generated higher IMEP with at an ignition timing slightly retarded from MBT than the gasoline fuel for a fixed throttle position. Under rich and stoichiometric conditions, the knock limited spark timing occurred at 35 CA BTDC whereas both ethanol and E90W10 were free from knocking combustion at the same operating condition. Heat release analysis reveals ethanol had the highest maximum cylinder pressure at the earliest crank angle, followed by E90W10 and then gasoline, because of their relative earlier combustion phasing (CA10 and CA50) and short combustion duration. This is confirmed by the faster developing flame fronts of ethanol fuels shown by the high speed combustion and OH* images. The difference in the flame radius among anhydrous, wet ethanol fuels and gasoline decreased when a fuel lean mixture was used. With the rich fuel/air mixture, the flame’s edges of all fuels became more wrinkled compared with that of stoichiometric and lean conditions.