The right ascension of 3C273B has been a reference of the celestial VLBI coordinate system. However it seems that 3C273B is inadequate for the reference because 3C273B has the jet component which was detected on long baselines in our experiments. We present the best reference source for celestial coordinate system. Next we estimated the source positions, taking the new reference radio source. The nutation and plate motion were considered in our analysis. We show the change of 3C273B position. The estimated source positions are compared with those given in GSFC source catalog (1985).