This paper reports the heat transfer coefficients in two-pass rotating rectangular channels [aspect ratio (AR=1:2 and AR=1:4)] with smooth walls. The experiments are conducted at four Reynolds numbers: 5000, 10,000, 25,000, and 40,000. The rotation numbers vary from 0.0 to 0.21 and 0.0 to 0.3 for AR=1:2 and AR=1:4, respectively. For each channel, two channel orientations are studied, 90 and 45 with respect to the plane of rotation. The results showed that the 180 sharp turn significantly enhanced heat transfer on both the leading and trailing surfaces in the turn region for both nonrotating and rotating channels. The results also showed that the rotation effect increased the heat transfer on the trailing surface in the first pass, but reduced the heat transfer on the leading surface. However, the heat transfer difference between the leading and trailing walls in the second pass is relatively small compared to the first pass due to strong turn effect.