The full-field endwall Nusselt number (Nu) distributions and flow field are presented respectively using steady-state infrared thermography and particle image velocimetry (PIV) for the twopass parallelogram channels with attached and detached transverse ribs. These square transverse ribs on two opposite channel endwalls are in-line arranged with rib-height to ductheight ratio of 0.1 and rib-pitch to rib-height ratio of 10. For the detached ribs, the detached distance between rib and channel endwall is 0.38 rib height. With the measurements of Fanning friction factor (f), heat transfer distributions and flow field features, the thermal performance factors (TPF) for the attached and detached rib cases are comparatively examined. A set of Nu, f and TPF results with the associated flow measurements at the test conditions of 5,000<Re<20,000 is selected to disclose the differential heat transfer enhancement mechanisms and heat transfer efficiencies between the attached and detached ribbed channels. Empirical correlations evaluating the endwall areaaveraged Nusselt numbers (Nu) and f factors are devised to assist the relevant design activities. Keywords: Parallelogram Two-Pass Ribbed Channel, PIV, Infrared Thermography, Turbine Blade Cooling.