The full-field Nusselt number (Nu) distributions and flow fields are presented, respectively, using steady-state infrared thermography (IR) and particle image velocimetry (PIV) in the two-pass parallelogram channels with attached and detached transverse ribs. These square transverse ribs on two opposite channel walls are in-line arranged with rib-height to duct-height ratio of 0.1 and rib pitch-to-height ratio of 10. For the detached ribs, the detached distance between rib and channel wall is 0.38 rib height. With the measurements of Fanning friction factor (f), the thermal performance factors (TPF) for the attached and detached-rib cases are comparatively examined. A set of Nu, f, and TPF with the associated flow measurements at the Reynolds number range of 5000 ≤ Re ≤ 20,000 is selected to disclose the differential heat transfer mechanisms and efficiencies between the attached and detached ribbed channels. Empirical correlations evaluating the area-averaged Nusselt numbers () and f factors are devised to assist the relevant design activities.