  The presence of a heavy fermion doublet of fourth generation in the standard
model would lead to a anomalous decay with fermion number non-conservation. The
anomalous decay path in the background of the electroweak instanton is
demonstrated by numerical calculation. If the mass of fermion exceeds the
critical value $m_f^{\rm cr}$=9.2 TeV (for $M_H=M_W$), it is shown that there
exists the rapid anomalous decay of the heavy fermion in semi-classical
approximation. The dependence of the critical fermion mass on the Higgs mass is
also presented.
