To reduce emissions from diesel engines, the effects of oxidation catalysts on the emissions reductions were studied. The effectiveness of several oxidation catalysts on both the regulated and unregulated emissions was evaluated. The oxidation activity of the catalysts was varied by changing Pt loading. The regulated emissions include particulate (PM), hydrocarbon (HC), and carbon monoxide (CO), and the unregulated emissions include benzene, formaldehyde, acetaldehyde, and benzo[a]pyrene (B[a]P). An 8 litter, turbocharged and aftercooled diesel engine was operated under the Japan Diesel 13 (D13) mode cycle for the evaluations.As the first step, evaluations were conducted with a commercially available JIS #2 diesel fuel (0.046 wt% sulfur). All the regulated and unregulated emissions except PM were reduced as the Pt loading (i.e. oxidation activity) increased. However, PM emissions were increased by the generation of sulfate when the Pt loading exceeded 0.2 g/l. The catalysts of low Pt loadings were less effective in reducing HC, CO, and the unregulated emissions although no sulfate generation was detected.Then, as the second step, the combinations of several oxidation catalysts of various Pt loadings and low sulfur diesel fuels (within 0.046 wt% sulfur) were tested to evaluate if PM can be reduced without the sulfate generation. The low sulfur diesel fuels were effective to reduce PM without sulfate generation regardless of the oxidation activity of catalysts. It was demonstrated that PM, CO, HC, benzene, formaldehyde, acetaldehyde, and B[a]P emissions were significantly reduced simultaneously by using an adequate oxidation catalyst and low sulfur diesel fuel. The PM conversion efficiency of such oxidation catalysts should be further improved before vehicle with the catalysts are commercially available.