  We study $\omega\to\pi^+\pi^-$ decay up to including all orders of the chiral
expansion and one-loop level of mesons in formlism of chiral constituent quark
model. This G-parity forbidden decay is caused by $m_u\neq m_d$ and
electromagnetic interaction of mesons. We illustrate that in the formlism both
nonresonant contact interaction and $\rho$ resonance exchange contribute to
this process, and the contribution from $\rho$ resonance exchange is dominant.
We obtain that transition matrix element is
$<~\rho|H_{\rho\omega}|\omega>=[-(3956\pm 280)-(1697\pm 130)i]$MeV$^2$, and
isospin breaking parameter is $m_d-m_u=3.9\pm 0.22$MeV at energy scale $\mu\sim
m_\omega$.
