  We study the time evolution of lepton family number for neutrino which forms
SU(2) doublet with charged lepton. The lepton family number is defined through
a weak basis of SU(2) doublet in which the charged lepton mass matrix is a real
and diagonal one. The lepton family number carried by the neutrino is defined
with a left-handed current of the neutrino family. We study the time evolution
of the lepton family number operator for Majorana neutrino. To be definite, we
introduce the mass term at $t=0$ and study the time evolution of the lepton
family number for the later time. Since the operator in flavor eigenstate is
continuously connected to that of the mass eigenstate, the creation and
annihilation operators for flavor eigenstates are related to those of mass
eigenstates. The total lepton number of the Majorana neutrino is conserved. By
choosing a specific flavor eigenstate of neutrino as an initial state, we
compute the time evolution of all lepton family numbers. They are sensitive to
Majorana and Dirac phases and also are sensitive to the absolute mass and mass
hierarchy of neutrinos.
