A mathematical model of accidental gas release in long transmission pipelines is given by using of computational fluid mechanics. It was found that the existing hole model is suitable for predicting gas release through a small punctured hole while the other existing pipe model is suitable to predict the gas release through a complete break in the pipe. In this paper, a new model was proposed for the hole that lies between the above two situations. The results of an example show that when the initial inside pipe pressure is higher than 1.5 MPa, the mass of gas released during sonic flow is more than 90% of the total mass of gas released. The total average release rate could be substituted by 30% of initial release rate. This approximation would become more accurate with increasing of the initial inside pipe pressure.