Electron-beam induced damage of some oxide superconductors is examined by high-resolution transmission electron microscopy (HRTEM), using both 1 MV high-voltage machine and conventional 200kV one. By 1 MV electron beam, both YBa2Cu4Oy and Y2Ba4Cu7Oy suffer from amorphization due to knock-on effects which is initiated at (CuO)2 double-chain sites. By 200kV beam, some of the double-chains decompose into the single-chains and precipitation of CuO crystal is induced, due to thermal heating effects. Both Bi- and Pb-based superconductors, on the other hand, suffer from amorphizations at both 1MV and 200kV. The materials relating to La2CuO4, for instance the T*-type of superconductor (Nd, Sr, Ce)2CuO4, never amorphize even by irradiations of 1000kV electron beam. These results suggest that it is possible to introduce defects or precipitates to control the microstructures of superconductors by means of electron beam irradiations.