Expansion of gas fired units installation in power network results in more interdependency between gas and electric infrastructures. Thus new research has paid attention to the impact of gas network in power system studies. One such issue is the impact of gas network in a unit commitment problem. Many linear and nonlinear models are proposed to demonstrate this impact. However, the dynamics of these two infrastructures are different. In contrary to an electric network, the dynamic of a gas network lasts over a long period. In large scale networks, the impact of gas volume variations may remain for some hours or even for a whole day. Earlier research considered a gas network static model by which its dynamic behavior was not observed. In this paper, the quasi dynamic model is proposed for a gas network by which gas velocity and distances between gas areas may be considered. This model is assessed for a unit commitment problem on a test network and its capabilities are appreciated. (C) 2011 Elsevier Ltd. All rights reserved.