Objective To observe the humidity absorption of different particle size of Rhizoma Coptidis.Methods The humidity absorption rate and the humidity absorption percent in normal powder

ultra-micro powder and nanometer dimension powder were determined on condition of 25℃ and 75% humidity absorption.Then its critical relative humidity can be determined by the balance curve of humidity absorption during the different humidity condition at 25℃.Results The humidity absorption rate and the humidity absorption percent in ultra-micro powder and nanometer dimension powder are higher than that in the normal powder.The critical ralative humidity in normal powder

ultra-micro powder and nanometer dimension powder are 72.3%、72.0% and 71.4% respectively.Conclusion The humidity absorption was changed with its particle size.