This paper experimentally studies the effects of the buoyancy force and channel aspect ratio (W:H) on heat transfer in two-pass rotating rectangular channels with smooth walls and 45deg ribbed walls. The channel aspect ratios include 4:1, 2:1, 1:1, 1:2, and 1:4. Four Reynolds numbers are studied: 5000, 10,000, 25,000, and 40,000. The rotation speed is fixed at 550rpm for all tests, and for each channel, two channel orientations are studied: 90deg and 45 or 135deg, with respect to the plane of rotation. The maximum inlet coolant-to-wall density ratio ()inlet is maintained around 0.12. Rib turbulators are placed on the leading and trailing walls of the channels at an angle of 45deg to the flow direction. The ribs have a 1.59 by 1.59mm square cross section, and the rib pitch-to-height ratio (Pe) is 10 for all tests. Under the fixed rotation speed (550rpm) and fixed inlet coolant-to-wall density ratio (0.12), the local buoyancy parameter is varied with different Reynolds numbers, local rotating radius, local coolant-to-wall density ratio, and channel hydraulic diameter. The effects of the local buoyancy parameter and channel aspect ratio on the regional Nusselt number ratio are presented. The results show that increasing the local buoyancy parameter increases the Nusselt number ratio on the trailing surface and decreases the Nusselt number ratio on the leading surface in the first pass for all channels. However, the trend of the Nusselt number ratio in the second pass is more complicated due to the strong effect of the 180deg turn. Results are also presented for this critical turn region of the two-pass channels. In addition to these regions, the channel averaged heat transfer, friction factor, and thermal performance are determined for each channel. With the channels having comparable Nusselt number ratios, the 1:4 channel has the superior thermal performance because it incurs the least pressure penalty.