  Issues of proton decay are examined in supersymmetric standard model with
U(1) flavor symmetry. Dimension five proton-decay operators which arise
generically are controlled by the flavor symmetry. We show that unlike the
minimal supersymmetric SU(5) case the proton decay modes containing charged
lepton can have large branching ratios if the dimension five operators of
left-handed type dominate. Measuring the branching ratio of the electron mode
to the muon mode may reveal the mechanism of the neutrino mass generation. The
case with vanishing charges for Higgs doublets marginally survives the present
experimental bound on proton life time.
