PM (Particulate Matter) 2.5 refers to microscopic solid or liquid particles with diameter of 2.5µm or less which suspend in the air. Nowadays PM2.5 pollution has become a serious problem in developing countries due to their intensive manufacturing and construction activities. Those particles are threating people’s health because PM2.5 can easily invade human lungs and penetrate into deep inner tissues inside human body. PM2.5 pollution is linked to increased lung cancer, heart attack and many other diseases. Monitoring the PM2.5 concentration in the air has become pressing need for people living in the area with serious air pollution. Conventional air quality sensors use beta ray attenuation to sense PM2.5 concentration in government apartment. Optical sensing is frequently used in portable PM2.5 sensors. In this poster, the design of an ionization-based portable PM2.5 air quality sensor is proposed. The device is not only able to detect the concentration of PM2.5 in the air, it also can analysis the amount of particles in the air when we change the ionized gas. The Monte Carlo method makes results closer to the situation we will face in the real lives, because diameters of particles are not the same. The particulate matter in the air sample is then ionized to become charged particles. Such ionized particulate matter will then be accelerated by an electrical field and finally hit the collection electrode. Double deflection can take segregation progress between charged PM2.5 particles and ionized nitrogen one step further.