An experiment was carried out on a four-cylinder spark-ignited (SI) gasoline engine to investigate the effect of hydrogen addition on the engine performance at low idle speed conditions. The engine was run at three idle speeds of 800r/min, 700r/min and 600r/min. At the specified idle speed and stoichiometric conditions, the hydrogen volumetric fraction in the total intake was gradually increased from 0 to about 6%. The experimental results showed that the engine fuel energy flow rate was reduced from 30.8MJ/h at the idle speed of 800r/min and a hydrogen volumetric fraction in the intake of 0 to 18.6MJ/h at the idle speed of 600r/min and a hydrogen volumetric fraction in the intake of 6.0%. For different idle speeds, the engine combustion duration was shortened, cyclic variation was eased and HC, CO and NOx emissions were decreased with the increase of hydrogen volumetric fraction in the intake. All in all, the addition of hydrogen was effective for enhancing the engine performance at low idle speed conditions.