Background: 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. This study is aimed at assessing factors associated with utilization of IMNCI protocol by nurses West Arsi Zone, Oromia Region, Ethiopia. Methods: An institutional based cross-sectional study was conducted on a total of 185 nurses who were included from health facilities selected using simple random sampling technique. 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, of which 131 (70.8%) were males. More than half 103 (55.7%) of the respondents was aged between 25-29 years. The mean (± SD) age was 26.65 ± (1.7) with the range of 20-43 years and 107 (57.8%) were diploma nurses. The overall IMNCI protocol utilization was 58.7%. In multivariate analysis, the odd of IMNCI utilizations among nurses who had attended IMNCI training were 2.76 times higher compared to nurses who had not attended IMNCI training [AOR=2.76, 95% CI:1.388, 5.51]. Nurses who had practice of always referring chart booklet during every case management process were three times more likely to utilize IMNCI protocols [AOR=2.95, 95% CI: 1.48, 5.89] compared to their counterparts. Conclusions: The proportion of IMNCI utilizations in the selected districts was low and less than the WHO recommendations. Training and frequent referring of chart booklet were found to significantly associate with the utilization of the IMNCI protocols. Therefore the emphasis should be given to the provision of IMNCI training to the nurse by the stakeholders and motivating the nurses to always refer chart booklet, in order to enhance proper utilization of the protocols.