With more stringent CO2 emission regulation in the world, Plug-in Hybrid Electric Vehicle (PHEV, also known as off-vehicle charging hybrid electric vehicle, OVC-HEV) plays a more important role in the current modern market, such as in China. At the same time, Real Driving Emission (RDE) was introduced in both Euro 6d and China 6b regulation, which covers more factors in the real driving practice including altitude, environment temperature, fuel quality, driving behaviors, and so on, which could potentially impact the pollutant emissions. Besides above mentioned, for PHEV, the state of charge (SOC) of the battery is also considered as one important factor, which could impact the engine load and emissions. Thus, this paper discusses some testing results of a PHEV (1.5L MPI engine) from a Chinese OEM, which was tested in the laboratory with different initial SOC points (charge sustaining - CS mode and charge depleting - CD mode) and different environment temperature (230C, 00C & -100C) as well to check their impact to the tail pipe particle number (PN) emission. In addition, one RDE simulation cycle with urban driving only was run at low environment temperature (00C) on the chassis dynamometer and the result of particle number emission above 23nm (PN23) is discussed according to the China 6b and Euro 6e requirement. Particle number emission above 10nm (PN10) from tail pipe is also measured per the new requirement in Euro 7 proposal and potential China 7 regulation. In the end, some relevant possible suggestions to meet the current and the future regulation are also discussed.