We investigated the effects of H-2 on the number concentration of particulate matter (PM) emissions from a diesel engine fitted with a low-pressure loop exhaust gas recirculation system. We used a 2.2-L four-cylinder direct-injection diesel engine satisfying EURO V regulations, and converted this engine to include an H-2 feed. The air/fuel (A/F) ratio was varied in the range of 21.9-45.5 and the brake mean effective pressure was varied in the range of 2-6 bars to control the O-2 concentration and in-cylinder temperature, both of which are significant for PM emissions. The number concentration of the emitted PM was measured using a scanning mobility particle sizer. We found that the emitted PM decreased by the addition of H-2 which caused the unburned gas temperature increased. Furthermore, the degree of reduction was larger as the A/F ratio, load, and H-2 energy fraction increased. However, with A/F ratios of less than 21.9, the addition of H-2 increased the number concentration of emitted PM which was attributed to the small O-2 concentration at these A/F ratios. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.