ackground: Challenges remain in the implementation of family-based health insurance (FHI) – the key strategy for achieving universal health coverage in Vietnam. Objectives: To explore the experiences of and factors influencing enrolment in FHI among informal sector workers (ISW) in a rural district in Vietnam. Methods: A cross-sectional study was conducted from January to June 2016 in Tien Du, a rural district in Bac Ninh province. Secondary data were extracted from provincial social insurance databases to identify the rate of FHI among ISW in the district. Qualitative data were collected from 55 participants: 20 semi-structured in-depth interviews (IDI) and 6 focus group discussions (FGD). Data were transcribed verbatim. Thematic analysis was conducted. Results: Only 25.1% of ISW had FHI. Those individuals with FHI tended to be older or already experiencing health problems or have ‘risky’ jobs. The key determinants of adverse selection from different levels included: individual, family and health system. Conclusions: Low health insurance uptake remains an issue for ISW. Policy makers should consider raising awareness of FHI and improve the quality of primary health services, simplify the procedures of enrolment and claim for ISW. Additional government premium subsidy is required to increase FHI enrolment among ISW.