Fatigue cracks have been observed at welded joints between U-ribs and deck plates in many orthotropic steel decks. Considering a fatigue crack at rib-to-deck welded joints can compromise transportation safety, it is necessary to detect and repair the crack at the early development stage.This paper presents the accuracy of crack sizing with a Phased Array Ultrasonic Testing (PAUT) scanner developed for inspection. Firstly, an internal crack propagating from a weld root was induced at rib-to-deck welded joints by using a plate bending fatigue testing machine. Subsequently, the joints were inspected with the PAUT scanner. Appropriate sizing methods were determined for the deck and weld bead propagation type cracks, respectively, and the accuracy of crack sizing was evaluated. In addition, the influence of coating on the accuracy in crack sizing was examined. As a result, it is demonstrated that the PAUT scanner had enough crack sizing performance.