Through this study, velocity distribution characteristics of exit section and downstream straight length of square sectional 180° and 90° curved ducts were numerically analyzed with RSM (Reynolds Stress Model) by changing various flow parameters such as air velocity, surface roughness and hydraulic diameter etc. As a result of it, the maximum velocity profiles according to air velocity in 180° and 90° curved ducts were indicated at Y/D=0.8 and X/D=0.7 region. Also, the efficient measuring location in bended duct downstream was able to suggest when the most stable straight duct length in flow deviations were indicated at L/D=30 and L/D=10 region, and the standard deviation characteristics according to hydraulic diameter were mostly indicated at L/D=15∼30 and L/D=10 region considering minimum deviations in the same velocity condition.