  We calculate the two-boson-exchange (TBE) corrections to the parity-violating
asymmetry of the elastic electron-proton scattering in a simple hadronic model
including the nucleon and the $\Delta(1232)$ intermediate states. We find that
$\Delta$ contribution $\delta_\Delta$ is, in general, comparable with the
nucleon contribution $\delta_N$ and the current experimental measurements of
strange-quark effects in the proton neutral weak current. The total TBE
corrections to the current extracted values of
  $G^{s}_{E}+\beta G^{s}_{M}$ in recent experiments are found to lie in the
range of $-7\sim +7%$.
