  We study the possibility of reheating the universe in its early stages
through the evaporation of Primordial Black Holes (PBHs) that are formed due to
the collapse of the inhomogeneities that were generated during inflation. By
using the current results of the baryon-photon ratio obtained from BBN and CMB
observations, we impose constraints on the spectral index of perturbations on
those small scales that cannot be estimated through CMB anisotropy and CMB
distortions. The masses of the PBHs constrained in this study lie in the range
of $10^{9}$ and $10^{11}$g, which corresponds to those PBHs whose maximal
evaporation took place during the redshifts $10^{6} < z < 10^{9}$. It is shown
that the upper bound on the scalar spectral index, $ n_{s}$ can be constrained
for a given threshold value, $ \zeta _{\rm th}$, of the curvature perturbations
for PBHs formation. Using Planck results for cosmological parameters we
obtained $n_{s} < 1.309 $ for $ \zeta _{\rm th} =0.7 $ and $ n_{s} < 1.334 $
for $ \zeta _{\rm th} = 1.2 $ respectively. The density fraction that has
contributed to the formation of Primordial Black Holes has also been estimated.
