Pressure recovery performances of circular and rectangular mixing ducts were investigated. Cold flow experiments were conducted using supersonic-subsonic ejectors. Afterburning in the second combustor was simulated by inserting plugs into exits of the mixing ducts. Maximum pressures in the mixing ducts and the pressure recovery lengths, namely, the distance from the rocket nozzle exit to the maximum pressure points, were measured when the mass flow rates of the breathed airflow and rocket exhaust were changed. The maximum pressures in the rectangular mixing ducts were about 5% lower than those of the circular ducts. However, the pressure recovery lengths of the rectangular ducts were 50% longer than those of the circular ducts. This may be due to the asymmetric flow in the rectangular ducts caused by the separations and the corner effects. Recovery performances of the short ducts, which were ten times longer than hydraulic diameters of each mixing duct, were investigated. More than 20% differences of the recovery performances were observed in the short ducts.