Singlet delta oxygen O2(a1Δ) molecules are generated by O2/He gaseous mixture dielectric barrier radio-frequency discharge. NO gas is added into main gas flow to improve the relative yield of O2(a1Δ). Influence of NO on the relative yield of O2(a1Δ) is examined by observing the O2(a1Δ) emission intensity as a function of NO flow rate. The experimental results show that the O2(a1Δ) relative yield reaches a saturation with the increase of NO flow rate and then slowly decreases. The optimal NO flow rate is about 3% of O2 flow rate under the conditions of the experiments. A new reaction mechanism including three-body oxygen atom recombination is proposed. According to the new mechanisms, a fitting function is given which agrees well with the experimental results. The rate constant of the new mechanism is obtained to be 1.8×10-30 cm6/s.