  We examine capabilities of the modified perturbation theory (MPT) for
description of the processes with productions and decays of fundamental
unstable particles. We calculate total cross-section for $e^{+} e^{-} \to
\gamma,Z \to W^{+} W^{-} \to 4f$ in a model with the Dyson-resummed and with
the MPT-expanded up to the NNLO Breit-Wigner factors, and compare the outcomes.
At the ILC energies a coincidence of the outcomes is detected with precision
better than one per-mille.
