This report evaluates the effect of thickness on the transformation behavior of Grade 91 steel alloy to further define the materials properties. The chemical compositions of six heats of Grade 91 steel were determined and two heats with furthest apart chemical composition were selected as test samples for the cooling rate tests and the Jominy hardness tests. A Jominy test apparatus was built in accordance with ASTM A255-10 and successfully tested. Six Jominy bars were machined, three from each of the two heats. Thermocouples were welded at five centerline locations in radial holes drilled along the length of one bar from each heat. Jominy cooling rate determinations from 1940°F were made at the five locations from each heat. Jominy cooling rate determinations were essentially identical between the two instrumented bars. Two Jominy bars from each Grade 91 heat were austenitized at 1940°F and subjected to the Jominy test. Hardness per ASTM A255 was made on one bar, the second bar was tempered at 1410°F and then the hardness tests were made. Photomicrographs were made at the five locations in the Jominy bars where the thermocouples were located. All of the microstructures showed 100% martensite; there was no indication of transformation to a microstructure other than martensite.