In this paper, a new deck plate system, in which the top chords are infilled with high-performance cement-based material such as high-strength mortar and UHPC (ultra-high performance concrete), was proposed. The bending capacity of the proposed deck plate system at the construction stage was investigated by performing a four-point bending test on seven specimens with a net span of 4.6 m. Test parameters included the type of top chords, type and amount of infilled material, and existence of inner shear connectors. The load versus displacement curves were plotted for all the specimens, and their failure modes were identified. In addition, a theoretical strength estimation process was developed for the proposed deck plate system. The strength values calculated by the estimation process were compared with the test results and analyzed. This comparison showed that the proposed system retained the peak strength and initial stiffness at a much higher level than the conventional system, and the theoretical strength estimation process can predict the failure modes and peak strength of the proposed system accurately.