We propose a model for the spectral formation of Gamma Ray Burst (GRB) prompt emission, where the phenomenological Band function is usually applied to describe the GRB prompt emission. We suggest that the GRB prompt emission is mainly a result of the Comptonization of relatively soft photons of the star off electrons of a hot shell of plasma of temperature of the order of 109 K (or ~100 keV) that moves sub-relativistically with the bulk velocity substantially less than the speed of light. In this case, the Comptonization parameter is high and the interaction between a blackbody-like soft seed photon population and hot electrons leads to formation of a saturated Comptonization spectrum. We give an interpretation of the Amati relation between the intrinsic spectral peak photon energy and radiated energy or luminosity.