Particle generation of ammonium nitrate from gaseous nitric acid and ammonia at low concentrations (100-600 ppbv) was investigated using a flow-type reactor with a silanized inner wall for deactivation. The particle number concentration of the generated aerosols was maximum at a constant relative humidity (RH) of around 60% in experiments at different gas concentrations. The RH of 60% coincides with the deliquescence relative humidity (DRH) of ammonium nitrate, and the physical state of particles is thought to be different below and above DRH. The size distribution of the particles was greatly affected by the RH. The NH4NO3 content in the wet particles was measured and compared with the value derived from the hydration-dehydration cycle. This report provides a simple and convenient method to generate NH4NO3 particles.