An extended $k-varepsilon$ tuebulence model modified by considering the streamline curvature effect and standard $k-varepsilon$ turbulence model have been applied for three dimensional analysis of turbulece flow in a $90^{circ}$ curved duct. By comparision of the results with the experimental data, the modified extended $k-varepsilon$ model gave closer agreement with experimental data than the results from standard $k-varepsilon$ model owing to an extra time scale of the production rate and parameter describing effects of streamline curvature included in the dissipation rate equation.