The emissions reduction benefits of Fischer-Tropsch (FT) diesel fuel have been shown in several recent published studies in both engine testing and in-use vehicle testing. FT diesel fuel shows significant advantages in reducing regulated engine emissions over conventional diesel fuel primarily to: its zero sulfur specification, implying reduced particulate matter (PM) emissions, its relatively lower aromaticity, and its relatively high cetane rating. However, the actual effect of FT diesel formulation on the in-cylinder combustion characteristics of unmodified modern heavy-duty diesel engines is not well documented. As a result, a Navistar T444E (V8, 7.3 liter) engine, instrumented for in-cylinder pressure measurement, was installed on an engine dynamometer and subjected to steady-state emissions measurement using both conventional Federal low sulfur pump diesel and a natural gas-derived FT fuel. The in-cylinder combustion pressure traces obtained during the engine testing were analyzed to obtain several pressure-based variables including ignition delay, combustion duration, peak pressure, location of peak pressure, relative quantities of premix and diffusive burn heat release, indicated mean effective pressure, and location of one-half of the mass fraction burned.These parameters were chosen because of their well-known effects on regulated gaseous and particulate matter emissions. The emission variations between the two fuels tested were compared to the differences in the measured combustion characteristics, and an explanation for the lower NOx and PM emissions of FT was developed.