The linear stability of the ion-temperature-gradient (ITG) driven drift modes is investigated in an International Thermonuclear Experimental Reactor-like geometry using an advanced reactive fluid model and the ballooning mode formalism. The spectrum of stable and unstable modes and their real frequencies, growth rates and eigenfunctions are calculated for two specific magnetic flux surfaces. The effects of density and temperature gradients, temperature ratios, wave vector and geometrical quantities such as local magnetic shear (LMS), normal curvature, geodesic curvature and magnetic field on the ITG mode are discussed. It is found that the most unstable eigenfunction is extended and less unstable at the magnetic surface where global magnetic shear is reversed. Moreover, the role of positive LMS is found to be destabilizing at the reverse shear magnetic surface. However, at a positive global shear magnetic surface, the eigenmode is found to be more localized and more unstable, and its structure and stability are affected by the local behaviour of the geometrical quantities.