From the beginning of archaeological research in the 20th century in Iran, most of archaeologists tend to study and research on western and the central Zagros region of this country due to the archaeological importance of this region which resulted in carrying out many research and excavations by numerous foreign archaeological teams. An accurate investigation on archaeological research shows significance and dignity of Kermanshah province, especially when it comes to Gilan Gharb city on aforementioned historical period. This city poses a key role in the historical period because of its specific location and also situating in one of the communication axes with Mesopotamia and West Asia. Investigating on biological conditions of forming settlements can be done by relying on archaeological studies, utilizing their analysis and pattern that give a clear understanding of how these settlements have been formed in Parthian period. The present study examines and evaluates 29 Parthian sites located in the West Gilan Plain which were identified in the archaeological survey of the central part of Gilan Gharb. The main question of this research is how these settlements have been formed in that era and effective factors on their establishment. To answer this question, firstly, explored potteries from different sites were identified and relative chronology of the sites has done. Then by using GIS software, the Parthian sites of Gilan West has analysed and evaluated according to two natural and human geography factors. Then, a wide range of variables such as: amount of precipitation, rivers and environmental beds have been analysed. Regarding to these analyses, the key role of considered variables e.g. Altitude, temperature status and access to water resources have been revealed as significant factors in formation of mentioned settlements.