A new method to predict NOx emission index of turbulent diffusion flame is proposed. The procedure is as follows: (1) To calculate emission index as a function of scalar dissipation rate chi(q) at an apparent flame surface position of steady laminar counterflow diffusion flame by adopting the detailed kinetics of large mechanisms. (2) To calculate probability density function PDF of scalar dissipation rate at flame surface position in fuel jet turbulent diffusion flame by adopting the flame surface model of infinite reaction rate and by solving directly the time dependent Navier-Stokes equation and conserved scalar equation by using a finite difference method. (3) To take average of local indices in terms of the above PDF to yield emission index of the whole flame. In this way, the combination of these two types calculation, (1) and (2), makes it possible to predict the emission index of the turbulent diffusion flame. The paper will describe some results of the prediction for methane-air turbulent diffusion flame and their comparison with existing experimental data.