Rotation effects on heat transfer, pressure drop, and thermal performance in a smooth square U-duct have been experimentally investigated. The Reynolds number ranges from 10,000 to 70,000, and the highest Rotation number reaches to 2.08. The density ratio maintains around 0.14 in all working conditions. In current study, a new rotating pressure measurements system is designed and validated. And the Nusselt numbers, friction factors and thermal performances in current study agree well with that in open literature. The experimental results show that the rotating-to-stationary ratios of these three parameters correlate with Rotation number in a very good way. In general, Nusselt number ratio increases linearly with Rotation number, but the friction ratio is oscillating when Rotation number increases. More detailed analysis of rotation effects on heat transfer shows that rotation has most dominate effects on averaged Nusselt number at the sharp turn compared with two straight passages by reinforcing the impingements over there. Due to the influence of sharp turn, rotation effects in second passage become dominant at high Rotation numbers. The friction ratio data suggest that the rotation has more apparent effects on friction ratio in heated channel than unheated channel. The Buoyancy force plays a very important role in channel's friction factor. Generally, it reduces the friction resistance in a smooth U-duct especially at high Rotation numbers. (C) 2013 Elsevier Ltd. All rights reserved.