Recently, MILD combustion without preheated air was achieved using a parallel jet burner, in which the fuel and air streams can entrain the hot exhaust gas simultaneously before their mixing. In this manuscript, the mechanisms of the effect of the entrainment ratio on NO formation in the MILD combustion without preheated air were critically focused on using OPPDIF with simultaneously diluted and preheated oxidant and fuel (HDO/HDF), in addition the effects of the inlet temperature and equivalence ratio are discussed. The variations of the NO production index at (1) T-in = 1300 K with the range of Kv from I to 8 (2) phi=1 with the range of T-in from 1000 K to 1300 K (3) T-in = 1300 K with the range of phi from 0.4 to 1.6 were numerically investigated. The results indicate that the NO emission decreases sharply when Kv increases from 1 to 3. This is attributed to the decrease in the NO production rate of N + OH <=> NO + H, the reduction of HNO to NO and the conversion of NO to NO2. When the entrainment ratio increases from 1 to 8, the pyrolysis zone is depressed and the reaction zone is broadened, resulting in an increase in OH consumption. Therefore, N + OH <=> NO + H and HNO + OH <=> NO + H2O sharply decrease in the production of NO. The mutual conversion between NO2 and NO is significantly enhanced in the MILD combustion mode without the preheated air. Consequently, NO2 + H <=> NO + OH makes the largest contribution to NO formation in the reaction region of the MILD combustion. The contribution of N + O-2 <=> NO + O in NO formation increases because CH + N-2 <=> HCN + N plays an increasingly important role in NO formation. Moreover, both of the inlet temperature and equivalence ratio enhance the NO formation, CH + N-2 <=> HCN + N are responsible for this effects. (C) 2016 Elsevier Ltd. All rights reserved.