Background: Home and community-based implementation of focused antenatal care intervention packages have significantly reduced neonatal mortality particularly in resource constrained settings. However, due to evidence-practice gaps, thousands of neonates are still losing their lives every day mostly from preventable causes. This study aimed to assess focused antenatal care package intervention fidelity and its determinant factors in South Wollo Zone, Northeast Ethiopia. Methods: A cross-sectional study design was employed and a total of 898 women who gave birth in the last six months prior to data collection were included. Also 16 health extension workers working in health posts and 10 health posts for facility audits were included. Interviews and self-administered questionnaires were used to collect data from mothers and health extension workers. Facility audits were used to assess availability and functionality of drugs and supplies in the health posts to provide focused antenatal care. Mothers were asked whether or not the required level of care was provided. Health extension workers were provided self-administered questionnaires to assess socio-demographic characteristics, reception of training, facilitation strategies and ability to classify danger signs. Multilevel linear regression analysis was performed to identify individual and organizational levels factors influencing focused antenatal care package intervention fidelity. Results: Overall weighted average focused antenatal care package intervention fidelity (implemented as intended/planned) was 49.8% (95% CI: 47.7 – 51.8); of which 55.1% was implemented by health extension workers and 44.9% by skilled providers (nurses, midwives, health officers or medical doctors). Overall antenatal care coverage, irrespective of frequency, was 83.7% (95% CI: 81.3–86.1); 35.0% (95% CI: of 31.6 – 38.4) of them received at least four antenatal visits and 46 (6.1%, 95% CI: 4.4 - 7.8) received all recommended components of focused antenatal care. Previous pregnancy-related medical problems, paternal education, and implementation of supportive/facilitation strategies were found to be significant factors enhancing focused antenatal care package intervention fidelity.Conclusion: Focused antenatal care package intervention fidelity in the study area was low; this may imply that stagnation of neonatal mortality reduction might be partly due to the low level of focused antenatal care fidelity. Improving implementation of facilitation strategies is highly required to contribute in neonatal mortality reduction.