In order to meet stricter diesel engine emissions regulations, several researchers have investigated the feasibility of utilizing natural gas as an alternative engine fuel. This study reports the performance, gas-phase emissions, and particulate emissions of a single-cylinder prechamber type diesel engine modified to run on natural gas/diesel dual fuel. During dual fueling, up to 73 percent of the diesel fuel energy input was replaced with natural gas while the engine power output was maintained at levels similar to those obtained using diesel fuel alone.Thermal efficiency obtained was 38 percent lower for dual-fueling than for straight diesel fueling at one-quarter load. However, at full load the thermal efficiency obtained for dual fueling was similar to that for straight diesel fueling. Similar exhaust NOx concentrations were obtained at one-quarter load, but at higher loads NOx concentrations were higher for dual fueling. Furthermore, CO concentrations were considerably higher at all loads using dual-fuels. On the other hand, both SO2 and CO2 concentrations were reduced as a result of dual fueling. When operating in the dual-fuel mode, Bosch smoke numbers and particulate emissions were reduced by up to 32 percent and 50 percent at full load, respectively. Particulates moisture content and soluble organic fraction (SOF) for dual fueling were higher than those for straight diesel fueling.