  In this work, we calculate the decay rates of the OZI-forbidden processes
$J/\psi(\Upsilon)\to \pi\pi, \rho\pi$ at the order of the leading-twist
distribution amplitude. The process of $J/\psi(\Upsilon)\to \pi^+ \pi^-$
violates isospin conservation and the amplitude is explicitly proportional to
the isospin violation factor $m_u-m_d$, our numerical results on their decay
rates are consistent with the data. The process $J/\psi(\Upsilon)\to \rho\pi$
violates the hadronic helicity conservation and should be suppressed, as
indicated in literature, its decay rate can only be proportional to $m_q^2$ at
the order of leading twist. Our theoretical evaluation confirms this statement
that the theoretical evaluation on $\Gamma(J/\psi(\Upsilon)\to \rho\pi)$ is
almost one order smaller than the data unless the model parameters take certain
extreme values. It may imply that the sizable branching ratio of
$J/\psi(\Upsilon)\to \rho\pi$ should be explained by either higher twist
contributions or other mechanisms.
