The effects of fuel sulfur content and dilution conditions on diesel engine PM number emissions have been researched extensively through steady state testing. Most results show that the concentration of nuclei-mode particles emitted increases with fuel sulfur content. A few studies further observed that fuel sulfur content has little effect on the emissions of heavily-used engines. It has also been found that primary dilution conditions can have a large impact on the size and number distribution of the nuclei-mode particles. These effects, however, have not yet been fully understood through transient testing, the method used by governments worldwide to certify engines and regulate emissions, and a means of experimentation which generates realistic conditions of on-road vehicles by varying the load and speed of the engine. This study investigates the effects of fuel sulfur content and primary dilution on the size distributions and number concentrations of particles emitted from a heavily-used diesel engine under both steady state and transient operations. The primary dilution was accomplished by a CFV-CVS, a system which maintains proportional sampling throughout temperature excursions and is designed for engine emission certification. The steady state results show reasonable agreement with previous studies, while the transient results show a large reduction of nuclei-mode particles with the use of ULSD compared to LSD during testing with the higher primary dilution.