Background: Option B+ prevention of mother to child transmission (PMTCT) of HIV is the lifelong provision of antiretroviral therapy for all HIV-positive pregnant and breastfeeding women regardless of immune status. In Ethiopia, the overall mother to child transmission rate of HIV was 15.9%. Objective: This study aimed to assess Determinants of HIV infection among infants born to HIV positive women receiving option B+ Prevention of Mother to child transmission of HIV in Tigray Region, North Ethiopia Methods: Unmatched case-control study was conducted in Tigray Region from October/2016 to April/ 2017. A total of 43 cases and 129 controls were selected using simple random sampling technique. Multivariable logistic regression analysis was fitted to identify the factors associated with mother to child transmission of HIV at a P<0.05. Results: Rural residence [AOR (95%CI): 33.3(1.02,87.05)], WHO disease stage III [AOR (95%CI): 57.4(9.25,297.54)] and IV [AOR (95%CI): 78.9(12.64,345.62)] during initiation of ART, and a child with mouth ulcer during exclusive breast feeding [AOR (95%CI): 65(6.39, 456.23)], absence of mothers’ educational status [AOR (95%CI): 0.2(0.04, 0.35)], late time of ART initiation after HIV diagnosis [AOR (95%CI): 0.14(0.02,0.18)] and absence of HIV exposed infant (HEI) follow up visit [AOR (95%CI): 0.04(0.005, 0.09)] were the factors significantly associated with mother to child transmission of HIV. Conclusion: The determinant factors were identified and significantly associated with mother to child transmission of HIV. Health care providers should strengthen option B+ PMTCT services to eliminate the mother to child transmission of HIV.