Abstract: The purpose of the study was to assess the effect of project delays on cost overruns risk in large –scale infrastructure projects in Rwanda focusing on Bugesera International Airport. This research dissertation used descriptive research design on108 sample of respondent’s stakeholders of bugesera international airport. Data were collected using Structured Questionnaires to obtain primary data and documentary review for secondary data, research sample were randomly selected from a total of 150 bugesera Airport stakeholders. A sample of 108 respondents was taken to assure a 95% significant result this research was supported by four theories which are critical path theory, resource-based theory, Cost Escalation theory and Iron Triangle Theory. Specific objectives of this study were to examine the effect of delays in project initiation phase on land acquisition cost,effect of delays in planning phase on labor cost and effect of delays in implementation phase on material cost. Inferential statistical analyses were employed to summarize responses and to establish correlations between variables. Afterward the study found that delays in the initiation phase of project significantly increased land acquisition cost due to bureaucracy and land disputes. The study also found that delays in planning phase increased labor cost of the project and final the study found that the delays in Implementation phase increased material cost of the project. The research confirms that the delays in project affect the cost overruns of Bugesra International Airport project where the project budget planned at initial stage up to 2026 increased at 333% which confirms that bugesera international project faced cost overruns due to the delays in all stage. From the results, the study recommends the use efficient mitigation techniques for planning efficiency rather formalities for sustainability in large-scale infrastructure projects in Rwanda. Final the study concludes that there is a significate effect of project delays in initial stage, planning phase and implementation phase on cost overruns risk of Bugesera International Airport.