Today, due to increasing the population of large cities such as Tehran, the demand for housing, has been increased. The imbalance between supply and demand of housing has increased the price of housing which caused citizens to live in small apartments. This issue reduces the quality of life in these homes. The quality of the human environment has an enormous effect on his health. Most research in the field of health psychology has shown that, there is a direct relationship between the quality of the residential environment and social harms such as suicide, mental disorders, drug addiction and criminality. One of these qualitative factors is the quality of daylight. Being exposed to adequate daylight has an enormous effect on the health of body and the human spirit. Due to the high density and compressibility of buildings and also aerosols and air pollution that cause low access to daylight, attention to the appropriate use of daylight in the cities such as Tehran is more critical. On the other hand, in tall apartments, residents have not access to some open spaces such as yard and during the day, they live in closed space of apartment. Therefore, the design of windows as the only source of daylight are very important. So, the conditions of suitable daylight in residential areas and the way to create it, are considered in this article.The study area of this article is small apartments in Tehran. Because of the high price of housing in Tehran, these apartments are considered by many citizens. Regarding to the limitation in access to the yard and outdoor living space, attention to improvement of environmental quality, especially attention to daylight is more important.The determining standard characteristics of daylight in residential areas is one part of this research. In this field with studying credible sources and standards, required data is collected. The most important index is a daylight factor that is equal to the average daylight in interior space in the proportion to outdoor daylight. Appropriate daylight factor for bedroom, work and study rooms, living room and kitchen respectively is equal to 1%, 5%, 2.5% and 2%. Identifying the common spaces of small apartments in Tehran and theirs area, is the next step. For this purpose a number of small residential apartments in the various parts of Tehran were selected and the different dimensions of theirs spaces were investigated. Finally, to determine the proper size of windows, the daylight simulation and calculation software (DIALUX) has been used. In this software, Tehran's geographic information (latitude and longitude) and the status of air pollution in Tehran and the data related to glass of window (common windows that used in this apartments) such as transparency coefficient and the reflection coefficient of glass, were imported and according to the standard daylight factor of bedroom, living room and kitchen, for each of these spaces, the calculations have been performed and the optimal window size for each of these spaces in proportion to the internal surface area is presented.