  We study the lepton sector in the model based on the local gauge group
$SU(3)_c\otimes SU(3)_L\otimes U(1)_X$ which do not contain particles with
exotic electric charges. The seesaw mechanism and discrete $A_4$ symmetry are
introduced into the model to understand why neutrinos are especially light and
the observed pattern of neutrino mixing. The model provides a method for
obtaining the tri-bimaximal mixing matrix in the leading order. A non-zero
mixing angle $V_{e3}$ presents in the modified mixing matrix.
