All-optical universal logic gates are theoretically investigated and demonstrated in a single silicon microring resonator due to mode conversion. Mode conversion takes place at the switching speed of 0.2 ps and at ultrahigh quality factor (Q-factor) and requires low power. Simulated results obtained from the finite-difference-time-domain method verified our proposed model. The Q-factor for NOT, NAND, and NOR logic gates is noted as high as around 1500, 1500, and 2400, respectively. The design is simple and silicon-on-insulator compatible.