  The Majorana nature of neutrinos can be detected by the precise measurement
of muon decay. This possibility comes from the presence of charged Higgs boson
interaction for Majorana neutrinos. We study the effects of the neutrino Yukawa
interaction via charged Higgs bosons in muon decay processes such as $\mu\to
e\nu\bar{\nu}$ and $\mu\to e\gamma$. The Higgs triplet model with small vacuum
expectation value is of special importance whose neutrino Yukawa coupling can
affect significantly muon decays. External neutrino lines in the Feynman
diagrams of $\mu\to e\nu\bar{\nu}$ can be crossed because of its Majorana
nature. This fact provides the interference contribution between the W boson
exchange diagram and that of charged Higgs boson, which may be detectable in
near future experiments.
