  In the supersymmetric grand unified theories (SUSY GUTs), gauge bosons
associated with the unified gauge group induce proton decay. We investigate the
proton decay rate via the gauge bosons in the SUSY GUTs under the two
situations; one is with extra vector-like multiplets, and the other is with
heavy sfermions. It is found that the proton lifetime is significantly reduced
in the former case, while in the latter case it is slightly prolonged.
Determination of the particle contents and their mass spectrum below the GUT
scale is important to predict the proton lifetime. The proton decay searches
have started to access to the 10^16 GeV scale.
