Introduction: Hepatitis B virus is a viral infection which can cause both acute and chronic liver disease. Predominately, this virus transmitted through the mother-to-child route. It is an endemic disease in low-resource setting countries like Ethiopia. Objective: Therefore, this study was used to assess magnitude and associated factors of hepatitis B surface antigen among pregnant mothers, who had antenatal care in Attat Hospital. Method: The cross-sectional study was conducted from 1 September 2019 to 30 January 2020. The data were collected from serological testing in laboratory and interview in antenatal care on 422 pregnant mothers. The data were entered into Epi Data version 3.1 and exported to SPSS version 24.0 for further analysis. Variables on bivariate analysis at p < 0.25 were taken to multivariate analysis. A p value of less than 0.05 on multivariate analysis was considered significant. Result: From 438 total samples, 422 respondents were participated and the mean age of the participants was 25.4 years (standard deviation ± 4.85). Magnitude of hepatitis B surface antigen was 10.9% (95% confidence interval: 8.3, 14.0). Respondents whose occupation was in government or self-employed were 67% (adjusted odds ratio = 0.33, 95% confidence interval: 0.12, 0.91) and whose number of gravidity was 3 and below were 79% (adjusted odds ratio = 0.21, 95% confidence interval: 0.04, 0.68) less likely to have hepatitis B surface antigen. Respondents who respond that hepatitis B surface antigen has cured were 1.52 times (adjusted odds ratio = 1.52, 95% confidence interval: 1.7, 9.4) more likely to have hepatitis B surface antigen. Respondents who answered hepatitis B surface antigen has vaccine were 57% (adjusted odds ratio = 0.43, 95% confidence interval: 0.05, 0.75) less likely to have hepatitis B surface antigen. Conclusion: Magnitude of hepatitis B surface antigen among mothers who were on antenatal care in Attat Hospital was high compared to other studies. Being employed in government or self-employed, gravida 3 and below, those who think that the disease has vaccine and those who think hepatitis B surface antigen was curable were statistically significant factors. Health promotion should be an important recommendation to be done by the hospital.