  We propose a mechanism for the natural inflation with and without modulation
in the framework of type IIB string theory on toroidal orientifold or orbifold.
We explicitly construct the stabilization potential of complex structure,
dilaton and K\"ahler moduli, where one of the imaginary component of complex
structure moduli becomes light which is identified as the inflaton. The
inflaton potential is generated by the gaugino-condensation term which receives
the one-loop threshold corrections determined by the field value of complex
structure moduli and the axion decay constant of inflaton is enhanced by the
inverse of one-loop factor. We also find the threshold corrections can also
induce the modulations to the original scalar potential for the natural
inflation. Depending on these modulations, we can predict several sizes of
tensor-to-scalar ratio as well as the other cosmological observables reported
by WMAP, Planck and/or BICEP2 collaborations.
