In order to be able to meet the demand for housing by the city's rapidly growing population after the Second World War, some important state supported mass housing projects were realized in Istanbul between 1945-55. Built in form of semi-detached and row houses, Kosuyolu neighborhood is one of the pioneers of the western style mass housing settlements of this period. In today's metropolitan Istanbul, the settlement has changed considerably. The aim of our paper is to examine the changing process of Kosuyolu district with some analysis ranging from architectural to urban scales. The results of this process both at housing types and the settlement are evaluated with the reasons, which caused these changes.