The particulate matter emissions have seriously negative effect on the human health, especially the ultrafine particulates and an experiment study has been conducted to evaluate the particulate emissions characteristics in a diesel/natural gas dual-fuel engine. In this paper, the particulate mass (PM) and particulate number (PN) concentration as well as particulate size distributions (PSD) characteristics under different natural gas-air mixture state operation conditions are investigated. The earlier (−500°CA ATDC, −480°CA ATDC) and later (−260°CA ATDC, −240°CA ATDC) natural gas injection timings are applied to form a homogenous and stratification natural gas-air mixture and the air fuel ratio (AFR) are maintained at a constant value during experiments. The results indicated that the natural gas-air mixture state have significant influence on the combustion process. The PN, especially ultrafine particulate emissions, is sensitive to the natural gas-air mixture state and the stratified-like mixture is benefit to reduce the PN of smaller size under low load. Moreover, in the diesel/natural gas dual-fuel engine, over 60% of the PN emissions are the ultrafine particulate while it just contributes to no more than 10% in the PM emissions.