This study is based on 6-years measurements (2004-2010) of total aerosol number concentration and size distribution at Yonsei University located near the center of Seoul, South Korea. Total aerosol number concentration and cloud condensation nuclei number concentration at 0.6% supersaturation are in the range of 17838±6814 cm-3 and 5153±2636 cm-3. New particle formation (NPF) events were observed on 9.3% of the days. Here we present an interesting case of NPF event that occurred on a cloudy and rainy day. On this day, precursor gas (sulfur dioxide) concentration was lower than the monthly mean. However, aerosol concentration was low on this day due to precipitation. It seems that the low aerosol concentration due to precipitation crucially affected the NPF event. Comprehensive analysis will be made and presented at the conference.