For half a century in gas distribution networks, polyethylene pipes with advantages such as non-corrosion, ease of implementation and operation cost reduction considered as a replacement for metal pipes. Considering the Irrecoverable gas leakage events, leakage forecast has great importance and this forecast can be used as a strategy for designing gas pipeline iection programs. Because of the lack of research in the field of the forecast of polyethylene pipes leakage, in this thesis, the leakage degree will be forecasted by collection data proportional to the effective factors leakage in this type of pipes. For this purpose, 32 factors were identified as the primary factors affecting the leakage of these pipes. Finally, by examining the data and opinion of the experts, eight factors were selected as the final factors.