Statistics indicates that fertility is falling below the level of replacement. In parallel to this declining trend, we see an increase in the proportion of one-child families. This change in the family structure affects the quality of one-child life and, due to role limitation, the one-child socialization process has different conditions. Therefore, one-child as a social phenomenon must be studied precisely. The existing studies focus on quantitative explanations of the change in fertility behavior, and the meaning of these behaviors is less well-known. Also, few studies that have been performed on one-child, have "why" assumptions; it is important to identify the "Quiddity" of one-child. The current research, by adopting a phenomenological approach, describes the experience of one-child living in Tehran. The data were collected through a deep interview with twenty girls over the age of 18 years using with purposive sampling, analyzed by smith's six-step technique. Twelve themes were extracted from data analysis. The themes are: " reign in family, in regret of other positions, extreme expectations, loneliness: Just yourself and yourself, need for family support, in search of compensation, credit link, weakness in emotional relationships, destructive worries, parent's educative encounter, self-adjustment, and confrontation with stereotypes". The findings of this study could provide a clear picture of one-child lived experience. In a general conclusion, the lived experiences of subjects show that those who are the only child are usually different in terms of life skills and individual capabilities, and this difference often shows itself in extremity. It should also be noted that one-child, because of lack of conflict of interests, usually has more facilities and welfare leading to emergence of some specific themes listed above.