This paper investigated the effect of hydrogen (H2) addition on the exhaust emissions of a 2004 Mack MP7 355E diesel engine. As expected, the addition of H2 substantially reduced the emissions of particulate matter (PM), unburned hydrocarbon (HC), and carbon dioxide (CO2). However, the effect of H2 addition on the emissions of carbon monoxide (CO) and nitrogen oxide (NOx) depended on the amount of H2 added and the engine load. When preliminarily measured using the 13-mode European Stationary Cycle (ESC), the addition of 2% and 4% (vol. % in intake mixture) H2 significantly increased nitrogen dioxide (NO2) emissions and slightly reduced nitric oxide (NO) emissions. In comparison, the H2 addition only minimally affected NOx emissions. The detailed effects of H2 addition on the exhaust emissions were investigated for various loads at 1200 rpm. The addition of H2 at low load mildly influenced NOx emissions, with the exception of 10% load operation. Adding over 4% H2 at 10% load began to reduce NOx emissions. When operated at medium to high load, the addition of a relatively small amount of H2 slightly reduced NOx emissions. The expected increase in NOx emissions was observed only with the addition of a large amount of H2. When operated at full load, the addition of H2 had negligible effect on NOx emissions. In a few cases, a minor change in H2 flow rate suddenly increased the emissions of NOx. By further increasing or reducing the amount of H2 added, NOx emissions resumed to their expected values. This discrepancy was attributed to the unexpected change in EGR flow rate. Based on the data obtained, it seems infeasible to substantially reduce the exhaust emissions of diesel engine through the addition of a small amount of H2 such as that produced on-board using a small H2O electrolyzer.