Although there have been several studies on the shear behavior of prestressed concrete beams, these generally focused on shear capacity rather than shear deformation. The present study investigated the shear deformation of large-scale reinforced I-shaped girders and post-tensioned prestressed concrete girders with a small shear span-depth ratio of 2.5. The test variables were the compressive strength of the concrete, the stirrup ratio, and the prestressing force. This large-scale experimental study enabled the investigation of diagonal cracking behavior, namely crack zones, patterns, principal strain direction, and crack width, as well as ultimate shear capacity. This extensive information can be used to establish the formation of compressive struts and force-resisting mechanisms in shear analysis. The experimental results show that the ultimate shear capacity of concrete girders increased with an increase in the concrete compressive strength, the stirrup ratio, and the prestressing force. The effect of concrete strength in the girders with stirrups and prestressing force, however, was not much as in those without stirrup and prestressing force. The stirrup was highly effective for controlling diagonal crack width, while the prestressing force is only effective at delaying cracking load. With the comparison of the crack zones of the test girders, the stirrup ratio was revealed as the dominant factor in the arch action of a beam member with a short shear span-depth ratio.