Fluid flows in rectangular duct systems are measured by using the laser-Doppler velocity meter and computed by commercial software of Star-CD for comparisons between flows. Three rectangular systems are investigated in this study: a rectangular duct with a 90 degree bent elbow, a rectangular duct with two branches, and a rectangular duct in the middle of which a circular cylinder is located. These investigations show that the numerical solutions satisfactorily predict design factors: for example, the K -factor for an elbowed duct, the distribution ratio of flow rates into each branch from a main duct, and the Nusselt number around the circular duct. However, there are some disagreements in the velocity profile and turbulent kinetic energy at each cross section of the duct systems.