A numerical study is conducted to grasp the flame structure and NO emissions for a wide range of oxy-fuel combustion (covering from air-blown combustion to pure oxygen combustion) and various mole fractions of recirculated CO2 in a CH4-O-2/N-2/CO2 counterflow diffusion flame. Special concern is given to the difference of the flame structure and NO emissions between air-blown combustion and oxy-fuel combustion w/o recirculated CO2 and is also focused on chemical effects of recirculated CO2. Air-blown combustion and oxy-fuel combustion without recirculated CO2 are shown to be considerably different in the flame structure and NO emissions. Modified fuel oxidation reaction pathways in oxy-fuel combustion are provided in detail compared to those in air-blown combustion without recirculated CO2. The formation and destruction of NO through Fenimore and thermal mechanisms are also compared for air-blown combustion and oxy-fuel combustion without recirculated CO2, and the role of the recirculated CO2 and its chemical effects are discussed. Importantly contributing reaction steps to the formation and destruction of NO are also estimated in oxy-fuel combustion in comparison to air-blown combustion.