The properties of optical dot gain of printings were researched. The differences in the influence of various dot structures on optical dot gain was quantitatively analyzed to explore the general law of optical dot gain. Based on dot structures, the definition of sensitiveness of circumference to dot area ratio was proposed, and the variation law of optical dot gain with dot structure characteristics was analyzed. Various dots were acquired through RIP. From the perspective of dot sensitiveness, the dot gain law was discussed and then demonstrated through offset printings. For AM dot, optical dot gain was more than physical dot gain. But for FM dot, physical dot gain was more than optical dot gain. Concentric dot owned special structures with properties of both AM and FM dot gain. Optical dot gain was inevitable and positively associated with the sensitiveness of circumference to dot area ratio. Research on the law of optical dot gain is significant to improve printing control.