The flame height of diffusion flames rising vertically from a cylindrical burner tube in quiet air extends with increasing the gas exit velocity and becomes constant in f fully developed turbulent region. The turbulence intensity at the burner exit which is measured by LDV increases linearly with the gas exit velocity for nonswirling flames. The mixing length at the burner exit calculated from the flame height and the turbulence intensity is shortened gradually with the gas velocity and then becomes constant for nonswirling flames. For swirling turbulent diffusion flames, the mixing length decreases with the swirl intensity at the burner exit. The coefficient of the tube diameter in the mixing length for burning flow is less than half the value of than the tube flow or round jet.