Abstract: Objective：To research the hydroscopicity of ultrafine powder addtive of anti-heat stress herbs for chicken. Methods：Made a comparison of the humidity absorption speed and rate between coarse powder and ultrafine powder additive；Critical relative humidity of coarse powder and ultrafine powder additive were determined；The storage conditions of ultrafine powder additive was optimized by orthogonal test with the temperature and humidity as factors using the humidity absorption percent as the index. Results：The humidity absorption percent in ultrafine powder additive was higher than that in the coarse powder. Critical relative humidity of ultrafine powder and coarse powder additive were 70％ and 78％ respectively. Compared with opening containing，the humidity absorption percent of ultrafine powder additive was significantly reduced after seal. The suitable storage conditions as follow：temperature 10 ℃，humidity below 75％. Conclusion：The hydroscopicity is changed after super-fine treatment，during the herbs processing，preparing and storing，it is quite necessary to control the environmental conditions.