Research on the capital structure of family firms has flourished in recent years, but the impact of performance aspiration and family ownership together on capital structure remains inadequately investigated. Therefore, the purpose of this study is to explore the impact of family ownership and under-aspiration performance and their interaction on capital structure. Panel data estimations were applied with a unique dataset of 3.857 observations from 387 public firms in Vietnam from 2010 to 2020 (134 family firms and 253 non-family firms). The results reveal that family ownership and under-aspiration performance each has a positive effect on capital structure. However, under-aspiration performance negatively moderates the positive effect of family ownership on capital structure. These findings contribute to a stream of studies on the capital structure of family firms by exploring the role of under-aspiration performance, as well as provide important implications for shareholders, managers and debtors in financial management.