Introduction: This study examines National Health Insurance Fund (NHIF) in Tanzania as a tool for improving universal coverage and accessibility to health care services was conducted in Dar es Salaam City. The main objective was to explore factors affecting accessibility to health care services and challenges in health care delivery by NHIF in Tanzania. Methodology: A multiple case cross-sectional design was employed and a sample of 58 respondents were selected using purposively and snowballing techniques in nine (9) NHIF accredited health facilities and NHIF head Office. A combination of qualitative and quantitative approaches was used in studying a phenomenon. Data were collected using documentary reviews, interview guides, focus group discussions (FGD) and questionnaires. Quantitative data from the questionnaires were analysed by using Statistical Package for Social Sciences (SPSS) and Microsoft Excel Spread Sheet, while qualitative data from interview guides and Focus Group Discussion were analysed descriptively using content analysis procedure. Findings: Although NHIF help to improved health care financing system in Tanzania, and provide capacity building to its staff, health facility administrators, NHIF focal persons as well as raises public awareness. The findings revealed that the reimbursement of NHIF bills to health care providers take long time contrary to NHIF reimbursement policy (within 60 working days) from when complains were tabled. Some of the NHIF benefits and packages services which are supposed to be provided to NHIF members are not provided. The results showed that among challenges faced by the NHIF members include; poor quality health services delivered, prolonged registration process, and most of health facilities favour much to the people who pay cash rather than the NHIF members. Recommendations: The study recommends that efforts should be made to improve quality of health services in the NHIF accredited health facilities. NHIF and health facilities providers must ensure that there is equal treatment between NHIF members and those who pay by cash. There is also a need for reviewing benefit and packages services and mechanism of reimbursement of NHIF bills.