In todayâ€™s global economy, organizations implement integrated software to create smooth inter-organization integration, get competitive advantage, hold customer satisfaction, produce real time report, provide user satisfaction as operating system, and getting output from the system. Enterprise resource planning (ERP) system is a set of integrated software modules and a central database that facilitates an organization to manage the efficient and effective use of resources. The effects of ERP system implementation on organizational performance needs to be known and measured so that appropriate & timely interventions can be undertaken for improvement. The main objectives of this study is to evaluate the effects of ERP system implementation on organizational performance in the selected public service companies in Ethiopia with respect to major ERP modules finance, human resource and sourcing & facility. The research adopted both explanatory and descriptive research designs with mixed of quantitative and qualitative research approach. Both primary and secondary data were used in the study for data collection purpose. The study population was composed of management and non-management staffs of selected public service companies, which were 1270 respondents. The study applied both stratified random sampling and purposive sampling techniques so as to allow in coming up with a sample size of 604 respondents, where primary data was collected directly from the respondents by the use of a questionnaire which was composed of both close ended and open ended questions. The collected data was analyzed using descriptive and inferential statistics using powerful econometric software STATA 11. Multiple regression analysis was used in the study to determine the effect of ERP system on organizational performance. The study results were presented by the use of tables, percentages and figures. The findings of the study show that financial ERP, human resource ERP and sourcing & facility ERP have statistically significant and positive effect on organizational performance of selected public service companies. In conclusion, the management of these companies as well as other organizations must emphasize on those variables and develop strategy accordingly for efficient and effective utilization of resources, managing time and expenses, facilitating organizational survival, profitability, maintain competitive advantage and generally improved overall organizational performance. Finally, the studyrnixrnrecommends that the companies must implement adequate training and development programs in order to enhance the user's satisfaction and be more productive on their job