There is a considerable difference in biological eligibility, for marriage, between males and females, which can make the age difference between spouses more prominent. In addition, variations in social norms and laws regarding marriage, through their effect on the conditions of family formation, can affect the age differences. The age difference can influence spouse's fecundity, the stability of marital union, marital satisfaction and the ideal number of children. Using the data from a survey on "socio-economic characteristics of households", conducted by Iranian Statistical Center in 2001, this paper aims to examine the factors affecting the age differences between spouses. For more than 85% of marriages, the age difference was in favor of men (older husbands). The mean age difference was positively related to men's age, urban residency, and levels of education, women's literacy rate and educational difference between spouses (in favor of men). For women, job outside the home, more freedom in spouse selection and age showed negative relationship with the age differences. The relation between the age differences and women's levels of education was arch-shape, being the greater differences for medium level of education. In the multivariate analysis, the significant effects of most individual independent variables were disappeared. This can reflect the sustainability of the cultural norms prescribing elder husbands across socio-economic differences.