Natural ventilation has the potential to reduce energy consumption and operating costs compared with mechanical ventilation. An opening for natural ventilation is carefully designed because of fluctuating wind velocity and direction. A pressure coefficient(Cp) is a dimensionless number which describes the relative pressures throughout a flow field. However, wind pressure coefficient values were results in above 4m/s wind velocity. The purpose of this study is to analyze the characteristics of pressure coefficient distribution in low wind velocity through a long-term measurement conducted from June 1st, 2010 to May 31st, 2011. The pressure difference, weather condition and airflow rates were measured using differential pressure sensor, weather station and multi-gas monitor. Consequently, in low wind velocity, it was found that there are significant differences of Cp between the result of measurement and AIVC database. Also, as wind velocity has increased, the fluctuation of Cp was small.