  The Imaging Air Cherenkov Telescopes (IACTs), like, HESS, MAGIC and VERITAS
well demonstrated their performances by showing many exciting results at very
high energy gamma ray domain, mainly between 100 GeV and 10 TeV. It is
important to investigate how much we can improve the sensitivity in this energy
range, but it is also important to expand the energy coverage and sensitivity
towards new domains, the lower and higher energies, by extending this IACT
techniques. For this purpose, we have carried out the optimization of the array
of large IACTs assuming with new technologies, advanced photodetectors, and
Ultra Fast readout system by Monte Carlo simulation, especially to obtain the
best sensitivity in the energy range between 10 GeV and 100 GeV. We will report
the performance of the array of Large IACTs with advanced technologies and its
limitation.
