Solid particle number emissions above and below 23 nm were investigated for a state-of-the-art stoichiometric heavy-duty natural gas engine with a three-way-catalyst and a diesel heavy-duty diesel engine with DFP. Furthermore, real time ash particle number emissions were measured using SwRI real time ash measurement instrument (RT-ASH). Both engines met the potential future CARB ultra low NOX emissions of 0.02 g/hp-hr, a 90% reduction from current emissions level in the USA. The engines were tested in an engine test cell under cold- and hot-start transient engine operation including FTP, WHTC and RMC. While both engines met comfortably the PM mass emissions, solid particle number emissions and ash emissions were significant for the natural gas engine. The emissions of solid particles from the natural gas was a factor of 5 to 10 higher than that of a diesel engine with DPF. Under WHTC testing, the natural gas engine showed more particles below 23 nm, compared to particles larger than 23 nm.