In this study, the impact of using 10% rapeseed methyl ester (RME) blended biodiesel fuel on the identity and quantity of the chemicals emitted by a modern diesel engine was investigated. The diesel engine that was utilized met Japan’s Post New Long Term emission regulations and was equipped with an after-treatment system comprising a diesel oxidation catalyst and a catalyzed diesel particulate filter (c-DPF). Exhaust gas sampling was conducted with three engine operating conditions, which were engine-out, tailpipe-out, and regeneration of the after-treatment system during the Japanese JE05 test cycles. Evidence from this study indicated that the use of 10% RME biodiesel had no significant impact on the regulated emissions of CO, CO2 total hydrocarbons, and NOx, regardless of the sampling conditions. However, some effects were evident on the emissions of elemental carbon, organic carbon, and polycyclic aromatic hydrocarbons (PAHs), which are unregulated. Under the engine-out and tailpipe-out conditions, emissions of the elemental carbon species EC2 associated with the use of 10% RME biodiesel were slightly lower than those associated with the use of petroleum diesel (D) fuel; however, an increase in the organic carbon species OC1, OC2, and some PAHs was observed under regeneration conditions because of the sizable consumption of 10% RME biodiesel compared to D fuel. These results from our study confirmed that 10% RME blended biodiesel is a promising alternative to fossil fuels for diesel engines, but it is important to grasp the behavior of such detailed components and counter the impact of increase in mixing ratios.