Mixing of fuel and air and NO formation are investigated numerically in a combustor with swirl-premix burners, which have been widely adopted in a gas turbine combustor. Numerical analyses of RANS are conducted in seven cases depending on the fuel-injector type. First, from the cold-flow numerical simulations, mixing of fuel and air is examined by the parameters of unmixedness and mixedness. It is predicted that NOx is reduced in the cases where mixedness is improved. Next, numerical analyses considering chemical reactions are conducted with boundary conditions of adiabatic and non-adiabatic conditions at the combustor walls, respectively. In numerical simulations for adiabatic condition, NO reduction induced by lance injection of fuel is not observed although it was in the experimental test. On the other hand, numerical results for non-adiabatic condition agree qualitatively with those from the experiments with respect to NO emission..