In the present research, combination of Indexing system Method with Analytical Hierarchy Process has been applied to assess the environmental risks of gas pipe lines. By this process, classification and qualification of the numerous types of environmental risks would be accessible. Sum Index and Leak Impact Index indicate risk probability and effect severity, respectively. In this regard, total environmental risk is calculated based on multiplication of total risk probability in effect severity. Analytical hierarchy process is applied to evaluate the factors because of differences existed in the total effective level of these factors. For this purpose, Tasooj- Salmas gas transfer pipe line, 24 inch in size and 42 km in length was selected to study the environmental risks. By using geographical information system, investigated risks have been classified throughout the pipe line route….