This study explores the marriage matching of only-child individuals and its outcome. Specifically, we analyze two aspects. First, we investigate how marital status (i.e., marriage with an only child, that with a non-only child and remaining single) differs between only children and non-only children. This analysis allows us to know whether people choose mates in a positive or a negative assortative manner regarding only-child status, and to predict whether only-child individuals benefit from marriage matching premiums or are subject to penalties regarding partner attractiveness. Second, we measure the premium/penalty by the size of the gap in partner's socio economic status (SES, here, years of schooling) between only-child and non--only-child individuals. The conventional economic theory and the observed marriage patterns of positive assortative mating on only-child status predict that only-child individuals are subject to a matching penalty in the marriage market, especially when their partner is also an only child. Furthermore, our estimation confirms that among especially women marrying an only-child husband, only children are penalized in terms of 0.57-years-lower educational attainment on the part of the partner.