  This paper is designed to discuss four-body lepton number violating tau
decay. We study the processes $\tau^+ \to e^+ e^+ \pi^- \nu_\tau$ and $\tau^+
\to e^+ e^+ \pi^- \nu_e$ to determine the nature of neutrino. The first process
violates lepton number by two units which can only happen through an internal
Majorana. The second one conserves lepton number but violates lepton flavor
which can take place with both Majorana neutrino and Dirac neutrino. We
calculate their branching ratio $Br$ and differential branching ratio
$dBr/dE_\pi$ to distinguish Majorana neutrino vs. Dirac neutrino. We also
propose the possibility of experiment to perform this detection.
