The Optical Variability of 3C273

B-band measurements of 3C273 over some 110 years are compiled and used in a search for periodicities using the Jurkevich method. Periods of 2.0, 13.65±0.20 and 22.5±0.2yr are found. If the long-term periodicity is from the instability of a slim disk, then the periodicity ( 13-yr or 22-yr) suggests masses of 10 7 M for the central black holes.