Dissertation (MSc Midwifery)Mara region is among the regions in Tanzania reported to have high maternal mortality ratios. Antenatal care services which are considered to prevent maternal deaths are under-utilized and the factors associated with under-utilization are not explained. Therefore, this study aimed to use health belief model and self-determination theory to explain ANC services utilization among postnatal women in Mara Region. A hospital based cross-sectional study involving postnatal women from April to July 2020 was done using a structured questionnaire. A total of 384 postnatal women were recruited using systematic sampling technique. Data were analyzed using SPSS version 20. Chi square was used to compare categorical variables. Univariate and multivariate analysis were performed to determine the association of variables. A p value of<0.05 was considered as statistically significant. Out of 384 respondents, 53.4% were self-determined, 46.9% were having high level of perception on ANC services and 49.7% had adequate knowledge on ANC services. About 77.1% booked ANC services late, and 85.9% attended less than eight recommended ANC visits. Being from Kurya ethnic group [AOR=0.428 (CI: 0.236-0.779) P = 0.005] and multiparity [AOR = 2.315 (CI: 1.256 - 4.266) P = 0.007] were the predictors of maternal self-determination. Multivariate analysis indicated that those women with low autonomy [AOR = 2.137 (CI: 1.185 - 3.852) P = 0.012], multiparty [AOR = .630 (CI: 1.244 - 5.59) P = 0.011], booking as a regular visit [AOR = 2.446 (CI: 1.210-4.946) P=0.013], self-employment [AOR = 5.089 (95% CI: 1.404 -18.447) P = 0.013] and unemployment [AOR = 5.256 (95% CI: 1.464 - 18.869) P = 0.011] were the predictors for ANC booking. Moreover, those women with low autonomy [AO R= 2.232 (1.045 - 4.767 P = 0.038], and low perceived barriers [AOR = 2.373 (95% CI: 1.267 - 4.447) P = 0.007] were the only predictors for ANC visits. ANC services are still inadequately utilized in Mara region. Low perceived autonomy and low perceived barriers showed to have significant association with inadequate ANC services. Other factors which showed significant association were ANC parity, and initiating ANC services as a regular cause. Efforts should be taken to improve womens self-determination (autonomy), and remove barriers so that to uphold the coverage of ANC services.