The positron lifetime spectra of the YBa2Cu3-xSnxOy system have been measured and the local electron density ne and vacancy concentration Cv have been calculated as a function of Sn substitution x. The results show that a sigificant Sn-substitution dependence exists. For lower x（<0.4）

ne has a big continual variation and for higher x（>0.8）

it is level off with a value of about 5.6×1023cm-3. But for Cv a big anomalous change occurs between x=0.8-1.0 and this may be the attributed to the structural phase transition.