Abstract: A detailed process concerning the detection of △9-THC in addict's hair is reported in this paper. It included washing and hydrolysis for hair specimen,as well as solid phase extraction,derivatization on disc and selection ion monitoring for the target' The recovery yield of THC for each step-i. e. extraction from hair,hydrolysis of hair specimen and derivatization of the target on SPE disc was determined quantitatively by GC/SIMMS. The method was applied to a case of the detection of THC in Addict's hair of 10 mg amounts .