Abstract: The gasoline engines usually encounter the dropped thermal efficiency and adversely increased toxic emissions at part load conditions.This paper investigated the effect of standard hydroxygen addition on the performance of a gasoline engine.The standard hydroxygen is defined as the hydroxygen with a hydrogen-to-oxygen mole ratio of 2∶1.The experiment was carried out on a four cylinder gasoline engine equipped with an electronically controlled hydrogen and oxygen injection system.During the experiment, the volume fraction of the standard hydroxygen in the total intake was raised from 0 to 2% and 4% by adjusting the injection durations of hydrogen and oxygen, respectively.The gasoline injection duration was reduced to enable the global excess air ratio of in-cylinder charge to increase from 1.00 to 1.50.The test results showed that the addition of standard hydroxygen availed improving the engine fuel economy.The lowest brake specific energy consumption rate was decreased from 12.63 MJ/kW·h to 12.27 MJ/kW·h and 12.11 MJ/kW·h of the 2% and 4% standard hydroxygen-blended gasoline engines, respectively.Moreover, it was also found that the flame development and propagation periods were shortened, the peak cylinder pressure rise rate was enhanced, HC and CO emissions were reduced whereas NO_x emissions were increased after the standard hydroxygen addition.