The influence of N 2 and He ratios in laser gas mixture on the pulse shape and onset delay time of the 10P(20) and 9P(20) lines in a bifurcated dual-band two-line TEA CO 2 laser was investigated. It was shown that the pulse shape as well as delay in onset time of 9P(20) line is strongly dependent on the ratios of N 2 and He components. So that, duration and onset delay time of the 9P(20) line can be respectively controlled in 100-400 ns range and 0.1 up to several ls, by varying the N 2 :He ratio in the gas mixture.