Based on weak equivalence principle (WEP) and the basic principle of quantum mechanics, we introduce concepts of the quantum of gravitational charge, the quantum level for gravity, and the expression for the quantum of energy of gravitational fields. Combination of these concepts with the dual geometric-gauge field aspects of gravity shows the following: ( 1) the quantum of gravitational charge corresponds to the quantum of the minimal energy and is the coupling constant in gauge field aspect of gravity; (2) When geometric aspect converts to gauge field aspect, spacetime translation of geometric gravity converts to internal symmetries of gauge gravity, the spacetime of geometric gravity converts to internal spacetime, and the metric function h(munu) converts to the gauge potential h(amu). The gauge potential h(amu), thus, is a spin one field; (3) Gravity should be quantized at the minimal energy quantum level, rather than the elementary particle level; (4) the self-interaction related difficulties in quantazition of gravity. the puzzle of the dual role played by the h(munu), and the nonlocalization of t(munu), may be resolved.
