The source processes of the 1965, 1968, and 1978 Oaxaca earthquakes are studied using teleseismic short-period records which contain more detail of the high-frequency component of the source-rupture process than long-period seismograms. The locations and the fault mechanism solutions of these earthquakes are similar and the recording stations are chosen common to the three earthquakes, so that the relative comparison of the source processes should be meaningful. The source process of each earthquake that was obtained is as follows: The 1965 event was a break of a single high-stress zone. The 1968 and 1978 events were complex breaks of several high-stress zones. The complexity of the source process has a good correlation with the foreshock activity (mb≥4.0) in the upcoming rupture zone, that is, the 1965 event had not foreshocks, the 1968 event had several foreshocks, and the 1978 even had one foreshock. For th e1965 event the moment value, 7¿1025 dyn cm, which is estimated for the single source is much smaller than that obtained from a surface wave analysis, that is, 2.2¿1027 dyn cm by Chael and Stewart.