The reburning chemistry in oxy-fuel and oxy-steam combustion of methane was investigated both experimentally and numerically. Comparison experiments in O-2/N-2, O-2/CO2, and O-2/H2O atmospheres were performed in a flow reactor at atmospheric pressure with equivalence ratio ranging from fuel-rich to fuel-lean and temperature from 973 to 1773 K. Experimental results showed that compared with N-2 and CO2 atmospheres NO reduction observed in H2O atmosphere is the lowest under fuel-rich and stoichiometric conditions, while it is the highest under fuel-lean conditions. The NO reduction intensity in CO2 atmosphere lies between N-2 and H2O atmosphere under fuel-rich and fuel-lean conditions; however, it is the highest under stoichiometric conditions. A chemical kinetic mechanism, which was hierarchically structured and updated in our previous work, captured the main characteristics and quantity of CO and NO formation satisfactorily even under fuel-lean conditions. According to the analysis from a chemical kinetic point of view, CO2 and H2O exert significant impacts on altering the radical pool structure to OH dominant, subsequently varying the availability of hydrocarbon radical as a reducing agent, which is the primary reason for the different degrees of NO reduction under fuel-rich, stoichiometric, and fuel-lean conditions. In addition, CO2 and H2O also impact the NO reduction by nitrogen-containing radicals. For CO2 atmosphere, NCO radical always occupies an overwhelmingly dominant position in NO reduction due to HCN -> CH3CN -> CH2CN -> CN -> NCO, and HNCO -> NCO channel is amplified substantially. For H2O atmosphere, under fuel-rich and stoichiometric conditions, NH2 and NH radical are dominant due to the enhancement of NCO -> HNCO -> NH2 -> NH channel. Under fuel-lean conditions, NCO radical is dominant due to the strength of HNCO -> NCO channel.