Solid particle emissions from a modern gasoline hybrid electrical vehicle (HEV) and a conventional gasoline vehicle were studied under diverse transient drive cycles on a chassis dynamometer.It is found that solid particle emissions from the conventional gasoline vehicle which has a 3.8-liter engine are sensitive to vehicle temperatures. Over 90% of solid particles are emitted during the first 250 seconds of transient cycles. Spikes for solid particle emissions from the HEV during a transient cycle are mainly caused by engine starts, hard accelerations after engine starts, and the engine running harder at high vehicle speeds. The engine and vehicle temperature status on the HEV doesn't show a strong correlation to the solid particle emission.While the conventional gasoline vehicle is cold, it emits over 2 times less solid particles than the HEV although it is larger and has heavier curb weight; and, while it is fully warmed up, it emits over 30 times less solid particles than the HEV. It is likely that frequent engine starts and a relatively higher percentage of maximum engine torque and power caused by engine downsize are contributing factors to increasing solid particle emissions from the HEV.