A construction project's success depends on its planning, accurate cost estimat-ing is critical in both public and private enterprises. Public building projects in Saudi Arabia have been plagued by enormous overruns and delays over the previ-ous three decades due to poor construction performance. The purpose of the re-search is determining the most critical factors affecting the accuracy of Saudi Ara-bian public project construction cost estimates. A questionnaire survey was given to 100 participants at random,95 of whom were males and 5 of whom were women, in order to acquire data. Cost estimates for Saudi public building projects are af-fected by financial concerns, the bidding situation and project features, as well as the estimation technique. The results found that the main factor related to financial issues. And the most significant factors of financial issues, which affecting cost estimates are financial status of client market, the unstable economic situation, cost estimation accuracy, financial resource availability, punctuality of cost flow as well as the availability of high quality financial plans and accurate estimation. Be-sides, the most significant factors of bidding situation, which affecting cost esti-mates are designer and design quality, company's strength and reputation in the construction industry, and amount of experience on such projects. Furthermore, the most significant factors of project characteristics, which affecting cost estimates are project manager competence, owners' requirement, Project specification and requirement, as well as project duration. Finally, the most significant factors of estimating process, which affecting cost estimates are project team's experience, time allowed for preparing estimates and availability of cost indices.