Abstract: The paper describes simultaneous determination of trace quantities of B,C and N in semiconductor silicon by nuclear reaction of 10B(d, n) 11C(T1/2=20.3min), 11B(d, 2n) 11C, 12C(d,n) 13N(T1/2 = 9.96min) and 14N(d, n) 15O(T1/2=2.03mm) with deutrons from a 1.2m cyclotron in our institute. An inert-gas fusion technique is adopted for rapid radiochemical separation after irradiation of the samples. 11C, 13N and 15O are absorbed in ascarite 5A molecular sieve cooled in liquid nitrogen and Hopcalite reagent at a temperature of 650℃ respectively. Positron annihilation events of each produced nucleus are counted by a γ-γ coincidence measuring system. B, C and N contents of about several ten parts per billion in silicon are then calculated simultaneously by a relatively quantitative method. Relative standard deviation for C, B and N are less than±50%,±20% and±50% respectively.This method is simple, rapid and sensitive for estimating light element content in silicon material.