Nitrated polycyclic aromatic hydrocarbons are more mutagenic and carcinogenic than parent PAHs. The formation mechanism of dinitro-pyrenes initiated by OH from mononitro-pyrenes was conducted by quantum chemical calculations. The products are 1,2-, 1,7-, 2,4-, 2,5- and 2,7- dinitropyrenes. Water molecules can decrease the Gibbs activation barriers of water loss step of NO2 -OH-nitropyrene. By canonical variational transition state theory with small curvature tunneling correction, the calculated overall rate constants for 1-, 2- and 4-nitropyrene are 8.13 x 10(-13) cm(3) molecule(-1) s(-1), 8.90 x 10(-13) cm(3) molecule(-1) s(-1) and 1.50 x 10(-12) cm(3) molecule(-1) s(-1) at 298 K and 1 atm, respectively.