A number of observation and modeling studies have been conducted in China to study the effectiveness of radical short-term emission control measures on air quality improvement. However, none of them have focused on the effect on submicron aerosols. Particle size distribution measurement down to 3 nm was conducted before, during and after the source emission control period for the 2016 G20 Summit held in Hangzhou, a megacity in eastern China. The measurement provided a unique opportunity to gain insights into the sources of submicron aerosols in Hangzhou and the effectiveness of radical emission control strategy. WRF-Chem simulation suggested that the sources inside the emission control region contributed more than 70% to PM1 in Hangzhou from September 2 to 7, 2016, even under the strictest control scheme. Source restrictions of coal-fired power plant emission, on-road vehicle and industrial VOC emissions resulted in concentration decreases of PM1, PM0.01 and PM0.1, respectively. New particle formation (NPF) event, which was not suppressed completely by the emission control, enhanced maximum PM0.01 concentration at around 11:00 AM by a factor of 4 as compared to that on non-NPF days. Even during the strictest emission control period, two PM1 episodes still occurred due to the dominance of subsidence airflow, low wind speed, and weak vertical temperature gradient.