The effects of mixture formation on soot were studied for diesel lift-off flame under low-temperature combustion (LTC) through comprehensive numerical analysis in this work. The predicted results of lift-off length, ignition time, and soot were validated against the experimental data under various ambient conditions. The results indicated that, when the O-2 concentration decreases gradually from 21 to 12% at 14.8 kg/m(3), the distribution ranges of both the fuel/air equivalence ratio (phi) and combustion temperature (T) of the mixture could be further away from the formation ranges of both high soot and high NOx. However, when the ambient density increased to 30 kg/m(3), the effect of a decreasing O-2 concentration on lowering the combustion temperature of the dense mixture within the range of phi > 2.0 was not significantly the same as that at 14.8 kg/m(3). Furthermore, the ignition time, the lift-off length, and the pressure rising rate during the initial stage after ignition changed much less with the decrease in the O-2 concentration. However, the LTC at 30 kg/m(3) could lower CO significantly and bring higher densities of OH and O-2, contributing to soot oxidation.