Relativistic Effects in S-Wave Quarkonium Decay

The decay widths of S-wave quarkonia ($\etc,\etb\to \gg{and} \J,\U\to\ee$) are calculated on the basis of a quasipotential approach. The nontrivial dependence on relative quark motion of decay amplitude is taken into consideration via quarkonium wave function. It is shown that relativistic corrections may be large (10-50 %) and comparable with QCD corrections.