There has been significant progress in expanding the use of contraceptives by women all over the world. However, despite considerable investment in family planning (FP) programs, the pace of improvement has been slow and regional disparities have been growing. The results of low use of family planning services led to high birth rates, bringing large families and economic overload. The study was conducted in Mafia District in Tanzania in order to examine the socio-cultural determinants of family planning practices among the people of reproductive age (15-45yrs). Specific objectives were to determine the knowledge level of respondents on family planning and to analyze socio-cultural factors affecting family planning practices. A cross-sectional research design was used. A total of 120 respondents including men and women were randomly selected from five villages. The respondents were selected based on propositional size to each village. Data were collected using a questionnaire survey. Statistical Package for Social science (SPSS) program was used to analyze the data. Descriptive and inferential statistics were used. Knowledge index score was used to compute respondent’s knowledge on family planning and binary logistic regression was used to identify factors affecting family planning practice. The study found that majority (95%) of the respondents had heard of FP but only (27.5%) used family planning services. The most common FP methods used were condom (86.7%) and injection (71.7%) the results based on multiple response. Majority (74.2%) accessed the FP services from MCH clinics. Socio-cultural factors mentioned were lack of knowledge, religious influence, local beliefs, and lack of information and partner’s opposition. However only religious and belief had significantly showed to be greatly (p<0.05) influencing the use of FP and local belief (p=0.037). Despite FP seemed to be well known, respondents knowledge was very low which most likely limited the contraceptive use. The study recommends that more education on the FP should be given to the community to increase more knowledge on the importance of the FP.