Periodic variations of the jet flow Lorentz factor in 3C 273

3C 273 has been observed with VLBI for more than 30 years. The entire data have shown that the position angle of the superluminal knots ejected from t he core varies periodically with a period of similar to 15 years. Moreover, their apparent velocity observed during the period 1963 - 1997 has systema tically decreased by a factor of greater than or equal to 2. These remarkab le properties are explained in terms of a precessing jet model, in which th e ejection Lorentz factor of the superluminal knots has been decreasing dur ing the last thirty years and has superposed on it a short-term (similar to 5 year) oscillation. The periodic variations derived by the model-fitting a re compared with the variations in the optical flux density. Binary black h ole models are briefly discussed to show possible relations of the observed periods to the periods involved in a binary system (orbital motion, spin o f black hole, accretion-disk rotation and Newtonian-driven precession etc.) .