  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.
