The effects of heavy ions on the slow shocks, which are considered to accompany the reconnection in the geomagnetotail, are investigated using one-dimensional hybrid code simulation (ion particle, charge neutralizing massless electron fluid). First, when all the ions are protons, the slow shock is found to have an ion cyclotron wave train in its downstream region in agreement with the two-fluid theory. This wave train, however, has not been observed by the spacecraft excursions in the geomagnetotail. In a case where we include oxygen ions in the lobe population with the density fraction as small as 2% of the total, the structure of slow shock deviates from the proton 100% case on the following two points. One is the disappearance of the wave train used to phase stand in the downstream region. The other is generation of the Alfv¿nic precursor, which is induced ahead of the slow shock and propagates upstream. This result suggests that the effect of a few percent of oxygen ions is one of the possible mechanisms which may explain the discrepancy between the two-fluid theory and observations. We propose the Alfv¿nic precursor generation mechanism and discuss its possible effects on the wave disruption. ¿ American Geophysical Union 1995