Spherical diffusion flame in microgravity was investigated computationally, considering gas radiation with statistical narrow band model (SNB) and discrete ordinate method (DOM). The parametric studies explored the relative effectiveness of fuel- versus oxidizer-side dilution on the flame radius and temperature behavior, and it was discovered that the oxidizer-side dilution has a stronger effect on flame transient behavior than the fuel-side dilution, thereby suggesting a more effective means to induce flame extinction by dilution. Study on different oxidizer-side dilution cases shows that CO2 has a larger suppression effect than helium and nitrogen with the same dilution level. CO2 dilution has multiple effects on flame behavior including radiation, thermodynamic, diffusion, and chemical effects. Quantitative analysis shows that the radiation effect is the primary factor accounting for flame temperature drop by approximately 60%, as compared to the thermal/diffusion (30%), and chemical effect (10%). Computational results over a wide range indicated a critical flame temperature of 1130 K at extinction, which appears to be a valid unified extinction criterion for the flame under study. Therefore, it is concluded that extinction of spherical diffusion flame is primarily dictated by the local condition in the flame zone rather than by the volumetric radiative heat transfer in the Surrounding gases. Investigation on steady flame solution within different domain sizes shows that, a steady state spherical diffusion flame does not exist in microgravity because the flame keeps growing with the non-zero gradient on the flame outer edge, additionally flame temperature constantly decreases with gas radiation which eventually extinguishes the flame when the flame temperature drops down to a critical value. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.