Background-Every year, nearly 11 million children die before reaching their fifth birthday. In response to this challenge, World health organization (WHO) and United Nations for Children‘s fund (UNICEF) in the early 1990s developed Integrated Management of Childhood Illness (IMCI), a strategy designed to reduce child mortality and morbidity in developing countries. Research that examines assessment of factors influencing the implementing the integrated management of neonatal and childhood illnesses (IMCI) strategy in Ethiopia is limited. Objective- To assess factors influencing the implementation of the IMNCI strategy by nursesin public health institutions of West Arsi zone, South West Ethiopia, 2016. Method- An institutional based cross-sectional studywas conducted from February to March. A total of 185 nurses were selected using proportionally allocated to population size andinterviewed using structured and pretestedquestionnaires. Data was coded, entered and cleaned using Epi-data 3.1 and exported to SPSS version 22 for analysis.Univariate and bivariate analysis was performed. Multivariate analysis was also done to control for possible confounding variables Results: Data were obtained from 185 nurses, yielding a response rate of 100%. The overall IMNCI implementation was 58.7% as high level implementation and 41.3% as low level implementation. In multivariate analysis the implementation of IMNCI was higher among IMNCI trained nurses ([AOR=2.76, 95%CI: 1.388, 5.51] and among those whose always referring chart booklet [AOR=2.95, 95%CI: 1.48, 5.89]. Conclusion:IMNCI strategy can be better implemented through provision of training for the health workers. However, a variety of factor found to be a barrier to IMNCI implementation in a consistent way. Recommendations have been made related to provision of the training to the nurses and HealthCare system strengthening among others. Keywords: Public heath instructions, Nurses, Integrated management of Neonatal and Childhood illness (IMNCI), Factors influencing the implementation of IMNCI,