An investigation has been made of modulational instability of a nonlinear ion acoustic wave in a weakly relativis-tic warm unmagnetized nonthermal plasma whose constituents are an inertial ion fluid and nonthermally distributedelectrons. Up to the second order of the perturbation theory, a nonlinear Schrodinger type (NST) equation for thecomplex amplitude of the perturbed ion density is obtained. The coefficients of this equation show that the relativisticeffect, the finite ion temperature and the nonthermal electrons modify the condition of the modulational stability. Theassociation between the small-wavenumber limit of the NST equation and the oscillatory solution of the Korteweg-deVaries equation, obtained by a reductive perturbation theory, is satisfied.