Heat transfer measurements using a transient liquid crystal thermometry (TLCT) are presented to study the effect of rotation on the local heat transfer distributions around a sharp 180 degrees turn of a two-pass smooth square duct for the first time in open literature. The duct has a dimensionless divider wall thickness of 0.25. Detailed local Nusselt number distributions on the leading and trailing walls are given for rotation numbers ranging from Ro = 0 to 0.20 at a Reynolds number of 1.0 x 10(4). The results show informative spanwise variations of heat transfer enhancement or abatement resulting from rotation. A critical range of Ro is identified, below which the regional averaged Nusselt number ratios of the regions after the 180 degrees sharp turn in a rotating two-pass cooling passage are rather insensitive to the rotation effect. Laser-Doppler velocimetry (LDV) measurements within the turn are further performed to explain the local heat transfer distributions. Under rotating conditions, the curvature induced symmetric Dean vortices in the turn for the stationary case change greatly and are dominated by a single vortex most of which impinges directly on the leading wall's outer part, resulting in a high heat transfer enhancement on the leading wall in the turn.