Combustion characteristics, flame structure, and length in an oxy-fuel combustor have been experimentally investigated for a wide range of fuel nozzle diameters and fuel and oxidizer velocities. Measurements are made for the model combustor (Combustor I) as well as for the actual oxy-fuel combustor (Combustor II). The experimental results for Combustor I clearly reveal basic features of the air-fuel and oxy-fuel flames in terms of flame length and structure. It was found that the flame length of the oxy-fuel flame in Combustor II decreases with increased fuel velocity or oxygen velocity due to greater turbulent mixing and entrainment. On the basis of the measured oxy-fuel flame lengths, a modified correlation between dimensionless flame lengths and a fire Froude number has been proposed for the two types of oxy-fuel combustors.