Introduction; Sunlight exposure is the main important source of vitamin D. Regular sunlight exposure increases the serotonin levels of human body, making it more active and alert. The most important advantage of exposure to sunlight is to increase the ability of the body boosting vitamin D supply. Objective: The aim of this study was to assess practice of sunlight exposure and associated factors of infants among mothers attending on immunization service in Bahir Dar City public health institution, North West, Ethiopia 2021 Methods: An institutional based cross sectional study was conducted among 443 mothers attending on immunization service from April 1 to May 30/2021 Systematic random sampling technique was used to collect data. The collected data was entered into Epi Info version7 software and exported to statically package for social science version 23 for analysis Both Bi-variate and multivariable logistic regression analysis was carried out. Those variables with P-value of less than 0.2 in bi-virate analysis were taken to multivariate analysis for farther analysis. Variables having P value of less than 0.05 with confidence level of 95% was considered as statistically significant Results: In this study 443 on immunization service attending mothers were eligible for interview and 414(93.5%) mothers were responded. Of these 311(75.1%) mothers were expose their infants to sunlight with 95% CI (71-79.2). Age group of mothers >31years[(AOR: 2.75,95%CI,(1.62-7.93)], maternal education diploma and above [(AOR: 6.75 95%CI: (1.723-26.433)], fear of sunlight exposure, [AOR: 0.23,95% CI, (0.13-0.48)], history of common cold [(AOR: 0.21, 95%CI, (0.15-0.45)]. and history of antenatal care follow-up [(AOR:2.46, (95% CI, (1.14-5.34)] were significantly associated with practices of sunlight exposure. Conclusion and recommendation: In this study practices of sunlight exposure of infants among immunization service attending mothers for their infants were low compared to other studies which is (75.1% [95%CI, (71-79.2)]. Maternal education, maternal age, history of fear of sunlight exposure, history of common cold of infants and history of Antenatal care follow up were significantly associated with practices of sunlight exposure. Counseling of mothers during Antenatal care, labor and delivery, postnatal care and at expanded program on immunization service about benefit of practices of sunlight exposure is recommended. Working in collaboration on behavioral change of mothers about sunlight exposure is recommended. Key words: - Immunization, mothers, infant, public health institution, Bahir Dar. Sunlight exposure