Damage of the urban gas network due to an earthquake can cause much loss including fire-induced loss to infrastructure and loss due to interruption of gas service and repairing or replacing of network elements. In this paper, a new fire ignition model is proposed and applied to a conventional semi- probabilistic model for estimating various losses due to damage of an urban gas network in an earthquake with the aim of developing a reliable tool for better designation of resources. The suggested fire ignition model takes into account parameters such as density of gas, characteristics of gas dispersion in a city, distribution of power lines as sources of ignition, and wind speed. Because of several parameters involved, inevitably a logical combination of deterministic and probabilistic variables is applied in the loss estimation model. Economic impacts of spreading of fire, gas service suspension, and gas network damage are modeled within the same semi-probabilistic framework utilizing weight functions. Assessing different fire scenarios is possible in the model for loss estimation. The model is applied to selected examples of actual urban area earthquake scenarios and the results are discussed.