In this article we investigated the modulation of nonlinear ion acoustic waves in a weakly relativistic, warm, unmagnetized and adiabatic plasma whose constitutes are ion fluid and superthermally distributed electrons using the multiple scales approach. The basic system of equations is reduced to a finite wave number nonlinear Schrödinger-type equation at the second order of the perturbation theory and for small wave number limit the nonlinear Schrödinger-type equation is derived. Moreover the reductive perturbation technique is applied to this system and Korteweg-de Vries equation is obtained. For small wavenumber limit, it is found that the dispersion coefficient and nonlinear coefficient of the nonlinear Schrödinger-type equation are reduced to the coefficients of nonlinear Schrödinger-type equation obtained from Korteweg-de Vries (K-dV) equation. The dependence of the phase velocity and the group as well as the domain of the stability and the instability on the temperature ratio, the relativistic factor u_0/(C ) and the superthermal parameter is investigated.