This research analyzes the extent of cost and time performance of public building construction projects in Addis Ababa and tries to identify the critical factors that influence cost and time performance of public building projects in Addis Ababa. Secondary data and questionnaire survey were used to collect relevant information. Secondary data analysis results shows that there is statistically significant variation in the cost and time performance of public building construction projects in Addis Ababa. The outcome of secondary data indicates that 74.3% of public building construction projects in Addis Ababa experienced cost under run and the remaining 25.7% projects were experienced cost overrun. Out of this 74.3% projects, 40% of the projects varies with more than negative 20%. Project time overrun of public projects is a critical problem of the construction industry. The research finding points out 100% of the projects delayed and not completed within the time stated. The minimum and the maximum time overrun is found to be 10% and above 250% respectively. Also to identify critical time and cost performance factors RII (relative importance index) value of each factors was calculated and for each parameters the top five critical factors were identified. Finally based on the findings and results some recommendations were given