Critical heat fluxes (CHFs) were measured on two horizontal flat plate heaters 6 mm wide and 20 mm long located on inner upper and lower walls of a rectangular duct at 50 mm from the inlet. The duct has 3 mm × 20 mm inner cross sectional area and 100 mm length. The flat plate heaters are made of Manganin and connected electrically in series. Therefore, the ratio of the cross-sectional area of duct to the heater area is 0.25. The measurements were made for the flow velocities from 0 m/s to 2.1 m/s and for the inlet liquid temperatures of 1.6 K, 1.8 K, 1.9 K, 2.0 K and 2.1 K. The measured CHFs were higher for larger flow velocity and lower liquid temperature. The equation of CHF for the forced convection was derived based on two fluid model, ordinary convection theorem and experimental results. It was confirmed that this correlation can describe not only the author's data on the duct but also other worker's data for channels with different shapes and sizes.