This study concerns the effects of diesel oxidation catalysts for heavy duty diesel engines over the Japan diesel 13 mode cycle. The effects of diesel oxidation catalysts were determined by measuring exhaust emissions from both a naturally aspirated diesel engine (NA-engine) and a turbo charged-after cooled diesel engine (TC-engine). The oxidation of particulate matter (PM), hydrocarbon (HC), carbon monoxide (GO) and the generation of sulfates were evaluated in the case of a palladium (Pd) based catalyst, a platinum (Pt) based catalyst and a base metal type catalyst, The effects of a high activity oxidation catalyst using ultra low sulfur diesel fuel were also reported. For the NA-engine, it was found that Pt based and Pd based oxidation catalysts had no effect on particulate reduction but reduced CO emissions and HC emissions, however, a base metal type oxidation catalyst had effects on particulate reduction but only small effects on the reduction of HC emissions and increased CO emissions. For the TC-engine, it was demonstrated that particulates, CO and HC emissions were reduced by Pd based oxidation catalysts and Pt based oxidation catalysts. When ultra low sulfur fuel was used, a high loading Pt based oxidation catalyst had a big effect to reduce particulates, HC and CO emissions for the NA-engine.