Premixed diesel combustion blending high volatility fuels into diesel fuel were investigated in a modern diesel engine. First, various fractions of normal heptane and diesel fuel were examined to determine the influence of the blending of a highly ignitable and volatile fuel into diesel fuel. The indicated thermal efficiency improves almost linearly with increasing normal heptane fraction, particularly at advanced injection timings when the fuel is not injected directly into the piston cavity. This improvement is mainly due to decreases in the other losses, ϕother which are calculated with the following equation based on the energy balance. ηu: The combustion efficiency calculated from the exhaust gas compositionsηi: The indicated thermal efficiencyϕex: The exhaust loss calculated from the enthalpy difference between intake and exhaust gasThe decreases in the other losses with normal heptane blends are due to a reduction in the unburned fuel which does not reach the gas analyzer.Next, a blend of 50 vol% iso-octane and 50 vol% diesel fuel was examined to determine the influence of blending a low ignitability and high volatility fuel similar to gasoline into diesel fuel. With the iso-octane blend the range of premixed diesel combustion can be expanded to higher intake oxygen concentrations due to a longer premixing duration with increasing ignition delay, and smokeless combustion is possible at low intake oxygen concentrations. The unburned fuel arising from adhesion of fuel spray on the cylinder wall can be avoided with the high volatility fuel, resulting in higher efficiency.