Thermal fluid characteristics of the lattice cooling channel have been investigated by measuring distributions of heat transfer coefficient. The heat transfer coefficient distributions have been obtained by visualizing steady-state local temperatures using temperature sensitive liquid crystal and foil heater. The measurement has been performed for all surfaces consisting of a single passage, while past studies focused only on the bottom (primary) surface. By comparing heat transfer coefficient distributions on the primary surface, sidewalls and turning regions with velocity field which was measured in the previous study, the heat transfer mechanism of the lattice cooling channel is discussed. The lattice cooling channel consists of two sets of inclined parallel ribs, which cross each other at right angles. Reynolds number is varied from 2,000 to 9,000, which is based on hydraulic diameter and bulk velocity in a sub-channel. Heat transfer patterns on the sub-channel before and after turning are compared. Overall, the heat transfer distribution was well-correlated with the velocity field. In the sub-channel before turning, heat transfer is enhanced at the entrance of the primary surface due to flow acceleration. Heat transfer is also enhanced on the sidewalls. Whether the coolant turns or not, shear force exerted by the crossing flow at the diamond-shaped openings generates swirl flow motion, leading to enhanced heat transfer. In the sub-channel after turning, this trend is further increased due to longitudinal vortex formed by the turning. Moreover, comparison of heat transfer patterns on the sidewalls with that on the primary surface suggested that they are comparable due to the flow interaction. It suggests that contribution of the rib surface on total heat release should be taken into account for reasonable cooling design. NOMENCLATURE A Area D hs Sub-channel hydraulic diameter H s Sub-channel height h Heat transfer coefficient L Streamwise distance from the sub-channel inlet Q Heat flux Nu Nusselt number Nu s Nusselt number of a smooth channel Pr Prandtl number Re s Sub-channel Reynolds number T a Air temperature T w Wall temperature V s Bulk velocity of sub-channel W s

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