Reducing idle speed is an effective way for decreasing engine idle fuel consumption. Unfortunately, due to the increased residual dilution and dropped combustion temperature, spark-ignited (SI) gasoline engines are prone to suffer high cyclic variation and even stall at low idle speeds. This paper investigated the effect of hydrogen addition on the performance of an SI gasoline engine at reduced idle speeds of 600, 700 and 800 rpm. The test results shows that cyclic variation was raised with the decrease of idle speed but reduced obviously with the increase of hydrogen energy fraction (beta(H2)). Decreasing idle speed and adding hydrogen were effective for reducing engine idle fuel consumption. The total fuel energy flow rate was effectively dropped from 30.8 MJ/h at 800 rpm and beta(H2) = 0% to 17.6 MJ/h at 600 rpm and beta(H2) = 19.9%. Because of the dropped fuel energy flow rate causing the reduced combustion temperature, both cooling and exhaust losses were markedly reduced after decreasing idle speed and adding hydrogen. HC and CO emissions were dropped with the increase of beta(H2), but increased after reducing idle speed. However, NOx emissions were decreased after reducing idle speed and adding hydrogen, due to the dropped peak cylinder temperature. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.