This paper presents an experimental study aimed at idle characteristics of a CNG engine fueled by HCNG with 55% hydrogen blend. The idle speed was reduced from original 800 r/min to 750 r/min and 700 r/min, and the characteristics of combustion & emissions at reduced idle speed were investigated. It is found that, for the HCNG engine, only reducing idle speed cannot reduce fuel consumption at conditions of fixed lambda. In order to reduce fuel consumption and keep the COV at rather low levels, the excess air ratio must be increased properly while reducing the engine idle speed. Due to the large valve overlap (30 degrees) of this inlet inject HCNG engine, CH4 emissions are mainly caused by scavenging, which account for the vast majority of THC emissions. The emissions of CO, THC and NOx are reduced with the decrease of ignition advance angle at a fixed lambda. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.