A three-dimensional numerical study of a constant property Newtonian fluid in 90 degrees strongly curved duct having a constant square cross section under steady and laminar flow conditions is presented for a uniform wall temperature boundary condition. Numerical solutions were obtained using commercial software Ansys Fluent 6.3.26 for the range of 165 <= De <= 1450 (250 <= Re <= 2200). The working fluid was air (Pr congruent to 0.7). The average Nusselt numbers and average Darcy friction factors are obtained, which can be used in estimation of flow and heat transfer performance in curved duct. In addition, local Nusselt numbers, local Darcy friction factors, velocity magnitude and temperature profiles, and secondary velocities are analyzed. Results are compared with the literature for validation of the numerical approach. It is seen that present results are in good agreement with the literature results.