In this paper, the radical induced (RI) ignition method was applied into a compressed natural gas (CNG) engine to achieve rapid bulk combustion. The experimental RI-CNG engine was modified from a diesel engine. The combustion chamber of the modified diesel engine was divided into a sub-chamber and a main-chamber. The sub-chamber is physically separated from the main-chamber above the piston and is connected to the main-chamber via several passage holes. CNG is injected into the sub-chamber during the intake stroke and then ignited before the top dead center (TDC) by a spark plug. As the ignition occurs in the sub-chamber, the pressure rises, forcing the gases which contain a number of active radicals out into the main-chamber to ignite the unburned mixture. The purpose of this paper is to study the engine operating limit and the combustion characteristics of the RI-CNG engine. The engine operating limit was accessed with different engine speeds and injection timings. The obtained data including the coefficient of variation (COV), brake specific fuel consumption (BSFC), mass fraction burned and emissions were analyzed.