The thermal recovery technology, SAGD, shows unique advantages to develop heavy oil reservoirs. A certain volume of non-condensate gas injected along with steam can effectively decrease heat loss from steam chamber to top-layer. Also, gas can dissolve in heavy oil to decrease the saturation of residual oil in the steam chamber. According to the one dimensional steady heat transfer theory, a mathematical model was introduced to optimize the thickness of gas layer during steam-gas assisted gravity drainage in heavy oil reservoir. Aiming at the dissolubility of non-condensate gas into heavy oil, a mathematical model was established to calculate injection rate of gas phase according to SAGD theory when steam and nitrogen were simultaneously injected into reservoirs. According to some geology characteristics, such as top water, large thickness, high viscosity, of an actual reservoir in China, the minimum thickness of nitrogen layer is 12.06 m, and the volume of injected nitrogen is 47.86 x 10(4) m(3), and the injection rate of nitrogen is 5957.96 m(3)/d when steam and nitrogen were simultaneously injected into reservoirs after SAGD stage. (C) 2017 Elsevier Ltd. All rights reserved.