  We study LHC signatures of Type III seesaw in which SU(2)_L triplet leptons
are introduced to supply the heavy seesaw masses. To detect the signals of
these heavy triplet leptons, one needs to understand their decays to standard
model particles which depend on how light and heavy leptons mix with each
other. We concentrate on the usual solutions with small light and heavy lepton
mixing of order the square root of the ratio of light and heavy masses,
(m_\nu/M_{\nu_R})^{1/2}. This class of solutions can lead to a visible
displaced vertex detectable at the LHC which can be used to distinguish small
mixing and large mixing between light and heavy leptons. We show that, in this
case, the couplings of light and heavy triplet leptons to gauge and Higgs
bosons, which determine the decay widths and branching ratios, can be expressed
in terms of light neutrino masses and their mixing. Using these relations, we
study heavy triplet lepton decay patterns and production cross section at the
LHC. If these heavy triplet leptons are below a TeV or so, they can be easily
produced at the LHC due to their gauge interactions from being non-trivial
representations of SU(2)_L. We consider two ideal production channels, 1)
E^+E^- \to \ell^+\ell^+ \ell^-\ell^- jj (\ell=e,\mu,\tau) and 2) E^\pm N \to
\ell^\pm \ell^\pm jjjj in detail. For case 1), we find that with one or two of
the light leptons being \tau it can also be effectively studied. With judicious
cuts at the LHC, the discovery of the heavy triplet leptons as high as a TeV
can be achieved with 100 fb^{-1} integrated luminosity.
