The present study investigated convective heat transfer inside a two-pass rectangular duct with guide vanes in the turning region. The objective was to determine the effect of the guide vanes on blade-tip cooling. The duct had a hydraulic diameter (Dh ) of 26.67 mm and an aspect ratio (AR) of 5. The duct inlet width was 80 mm, and the distance between the tip of the divider and the tip wall of the duct was also 80 mm. Various guide vane configurations were used in the turning region. The Reynolds number (Re), based on the hydraulic diameter, was held constant at 10,000. The naphthalene sublimation technique was used to determine the detailed local heat-transfer coefficients, using the heat-and mass-transfer analogy. The results indicate that guide vanes in the turning region enhance heat transfer in the blade-tip region. The guide vane on the second-pass side of the turning region had higher heat transfer than the guide vane on the first-pass side. Strong secondary flow enhanced heat transfer in the blade-tip region. Dean vortices induced by the guide vanes pushed the high-momentum core flow toward the tip wall, and heat transfer was increased in the turning region, but decreased in the second passage. Consequently, a guide vane on the second-pass side of the turning region generates high-heat-transfer rates on the tip surface, and can also increase the thermal performance factor in a two-pass duct.