  In the NRQCD factorization for productions and decays of a quakonium the
difference between the quarkonium mass and the twice of the heavy quark mass is
neglected at the leading order of the small velocity expansion. The effect of
this difference is included in the relativistic correction. We attempt to
define the difference in terms of NRQCD matrix elements and to separate the
effect of the difference in several decays. It turns out that the total
relativistic correcttion can be estimated by the difference. Numerically the
correction will enhance the decay widths considered here.
