A theoretical investigation of the HCNO + *OH reaction, which is carried out by means of B3L YP/6311++G(3df,2p) + ZPVE computational method to determine a set of reasonable pathways. The potential energy surface for this system shows that there are sixteen different product pathways: PRI (H2O + *CNO), PR2 (CH2O + NO), PR3 (OCHNO + H), PR4 (CHOH: + NO), PR5 (CO2 + "NH2), PR6 (HOCN + °OH), PR7 (OCNOH + H), PR8 (HOCNO + H), PR9 (OCN* + H20), PRI0 (H2 + OCNO), PRll (CO + *NHOH), PR12(OCNH + *OH), PR13 (CO + NO + H2), PR14 *CHO + HNO), PR15 (CO + NH2O*) and PR16 (HCN + HO2*). The results indicate that PR11 (CO + *NHOH) and PR14 (THO + HNO) are the major product channels. The rate constants for these processes have been calculated using variational RRKM theory covering the temperature range 300-1000 K and compared with available experimental data. The total rate constant of HCNO + *OH reaction at 1 atm, 300 K is 4.44 x 10 exponent -11 (cm3/molecule/s).