The flame structure of ethanol and nitrous oxide combustion was experimentally studied using a tricoaxial injector at various momentum flux ratios. The objects of the study were to investigate the effects of the additional supplement of nitrous oxide that gas jets inject at the outer annular gap at various injection velocities and to obtain and analyze the flame structure. The fully developed patterns due to the transfer of momentum and viscosity mixing were similar to that of the axial flow when the measurement position was increased from Z/d = 1 to Z/d = 10. The correlation equation, using the momentum flux ratio, was used along with the linear regression method to calculate the breakup length and Sauter mean diameter (SMD). The effects were clearly observed and significant. The inner gas injection caused the SMD to decrease, and the outer gas injection was able to create a boundary layer around the spray jets. As the momentum flux ratio of the inner gas jets increased, the spray angle and flame angle increased. OH radicals extended toward the rear flame region with the increase in momentum flux ratio from the inner gas jets. As the momentum flux of the outer gas jets increased, the boundary of the OH radicals developed and the intensity of the OH radicals generated in the flame region was enhanced. Also, the flame temperature increased as gas was injected from the outer-stage.