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During the recent decades, unorganized and informal settlements is gradually formed and expanded mainly in the outskirt of big cities, out of official plan of urban development. This phenomenon is considered as one of the urban dilema in the urban studies. In the process of urban pathology, one of the most important step is an accurate and comprehensive recognition of these dilema. In other words, only accurate recognition from the factors that created these dilama, could present the solution strategy in this regards. Effective recognition in the formation and development of squatter settlement is inevitable more than ever. This accurate and realistic recognition will be useful in two points. First, if an appropriate recognition to be achieved, suitable and illustrative observation would be available against the physical and non physical problems. Secondly, a suitable and appropriate solution to prevent in formation and expansion, also to improve the current condition of these areas. Urban study researchers have presented various reasons for the formation of squatters. According to the diversity of squatter phenomenon, we cannot generalize the reasons for its formation from one city to another. Karaj is one the metropolis of the country that providing various attractions as destination of many immigrants. In this paper, the reasons for formation of informal settlement and the process of its expansion has been studied in the Sohrabiye district of Karaj. This district has an area of about 33 hector and 5247 population and is located in the west of Karaj, next to the Ghezelhesar road.

  The methodology of the research in this paper has been analytical- descriptive and also analyzing the data resulted from the field survey. The mothod of sampling has been random and 1family from 10 families is selected. The result of this research shows that the origin of residence formation in the Sohrabiye district of Karaj has returned to the year of 1964. The first residents of this district have migrated following the land reformation. After Islamic Revolution (1979) and following the intensity process of migration in Karaj city and also variety advantages of Sohrabiye district for low income emigrants such as cheap land, Proximity to Karaj city and Ghezelhesar road as a connective axis of Sohrabiye district to main structure of Karaj city and lack of precise supervision of municipality on illegal construction. This district is selected as a destination emigration of many migrants. Also the result of research shows that near 85% of the emigrants who came to this district are come from western north and west of the country and their intension is employment. According to this, the necessity of fair distribution of facilities and providing job opportunities in these area could be as a main approach in preventing the expansion of illegal settlement in Sohrabiye district and city of karaj as whole.