Reinforced concrete thin Spherical dome shell roof structure construction for top covering ofrncontainment structures, memorial buildings, meeting halls, mosques and cathedral churches isrnin continuous demand in developing countries, like Ethiopia. Due to its strength, economy,rncontainment of large space and aesthetical value than other shape of roofs, spherical dome shellrnis largely constructed as a reinforced concrete roof structures. rnThis structure is classified as shell of revolution having greater than zero Gaussian curvaturernwhich shows the greatest stiffness of it. This roof structure can be subjected to axisymmetricrnand asymmetrical loadings, but in this study the axisymmetric loadings such as loads from toprnring beam, live load and self-weights are considered. rnIn this study, it has been investigated the cost optimized design of reinforced truncated sphericalrndome shell roof structure which is stiffened by rectangular cross section circular ring beam atrnbase of it. The cost optimized is done for spherical dome having base area from 78.539 mrn torn530.929 mrn2rn2rn using Scilab programing language. The developed program follows compatibilityrnmethod of analysis to get the effects of axisymmetrical external loadings, European codes forrndesigning of the sections and Brute-force optimization method to get the best minimum totalrnmaterial costs of the roofs. rn From this program the optimized total minimum material costs are obtained in addition tornoptimized values of cross sectional and geometrical dimensions, and reinforcement areas. Fromrnthe result most of the optimized design variables are obtained around the lower bounds of sidernconstraints. The optimized values of meridional reinforcements are obtained from singlyrnreinforced section design, and they have the same value per unit meter width at every meridionalrnangle. The circumferential reinforcements like the meridional have the same values per unitrnmeter at every meridional angle of spherical dome. The total cost spherical dome componentrnof the roof has significant influence on the total cost of the whole roof structure than ring beamrnstructural component and the material cost of the concrete has also significant influence on therntotal cost of the roof.