3C 273 has been observed with VLBI for more than 30 years. The entiredata have shown that the position angle of the superluminal knots ejected from the core varies periodically with a period of ～15 years. Moreover, their apparent velocity observed during the period 1963 - 1997 has systematically decreased by a factor of >～ 2. These remarkable properties are explained in terms of a precessing jet model, in which the ejection Lorentz factor of the superluminal knots has been decreasing during the last thirty years and has superposed on it a short-term (～5 year) oscillation. The periodic variations derived by the model-fitting are compared with the variations in the optical flux density. Binary black hole models are briefly discussed to show possible relations of the observed periods to the periods involved in a binary system (orbital motion, spin of black hole, accretion-disk rotation and Newtonian-driven precession etc.).