  We consider a scenario of cosmological inflation coming from a grand unified
theory in higher dimensional orbifold. Flatness of the potential is
automatically guaranteed in this orbifold setup thanks to the nonlocality of
the Wilson line on higher dimensions and the local quantum gravitational
corrections are exponentially suppressed. The spectral index of scalar
perturbation (n_s =0.92 - 0.97) and a significant production of gravitational
waves are predicted (r= T/S= 0.01 - 0.12) in the perturbative regime of gauge
interaction, (1/g_4 = (5-20) X 2 pi R Mpl) where the size of compactification
is constrained (R Mpl = 20- 45) by the measurement of scalar power spectrum
(Delta_R = 5 X 10^-5).
