The kinetics and mechanism of the reaction of the cyanomidyl radical (HNCN) with the hydroxyl radical (OH) have been investigated by ab initio calculations with rate constants prediction. The single and triplet potential energy surfaces of this reaction have been calculated by single-point calculations at the CCSD(T)/6-311+G(3df,2p) level based on geometries optimized at the B3LYP/6-311+G(3df,2p) and CCSD/6-311++ G(d,p) levels. The rate constants for various product channels in the temperature range of 300-3000 K are predicted by variational transition-state and Rice-Ramsperger-Kassel-Marcus (RRKM) theories. The predicted total rate constants can be represented by the expressions k(total) = 2.66 x 10(+2) x T-4.50 exp(-239/T) in which T = 300-1000 K and 1.38 x 10(-20) x T-2.78 exp(1578/T) cm(3) molecule(-1) s(-1) where T = 1000-3000 K. The branching ratios of primary channels are predicted: k(1) for forming singlet HON(H)CN accounts for 0.32-0.28, and k(4) for forming singlet HONCNH accounts for 0.68-0.17 in the temperature range of 300-800 K. k(2) + k(7) for producing H2O + NCN accounts for 0.55-0.99 in the high-temperature range of 800-3000 K. The branching ratios of k(3) for producing HCN + HNO, k(6) for producing H2N + NCO, k(8) for forming (HN)-H-3(OH)CN, k(9) for producing CNOH + (NH)-N-3, and k(5) + k(10) for producing NH2 + NCO are negligible. The rate constants for key individual product channels are provided in a table for different temperature and pressure conditions.