The reactants, products, and transition states of the CH2O + NO2 reaction on the ground electronic potential energy surface have been searched at both B3LYP/6-311+G(d,p) and MPW1PW91/6-311+G(3df,2p) levels of theory. The forward and reverse barriers are further improved by a modified Gaussian-2 method. The theoretical rate constants for the two most favorable reaction channels 1 and 2 producing CHO + cis-HONO and CHO + HNO2, respectively, have been calculated over the temperature range from 200 to 3000 K using the conventional and variational transition-state theory with quantum-mechanical tunneling corrections. The former product channel was found to be dominant below 1500 K, above which the latter becomes competitive. The predicted total rate constants for these two product channels can be presented by kt(T) = 8.35 × 10-11 T6.68 exp(-4182/T) cm3/(mol s). The predicted values, which include the significant effect of small curvature tunneling corrections, are in quantitative agreement with the available experimental data throughout the temperature range studied (390-1650 K).