Here we establish a fundamental principle to open a relatively large band gap for graphene by hydrogenation. Specifically, the large band gap can be obtained when the number of nonmagnetic sp2-substructures on graphene basal plane is maximized. The principle indicates unequivocally what additive patterning should be used to attain the largest band gap for a given addition coverage. According to this principle, the graphene band gap can be continuously tuned from 0 to 5.2 eV. These findings may open a door to create graphene-based nanosystems with desired band gaps, and will become a significant foundation for graphene nanotechnology applications.