A new dispersion relation for the ion temperature gradient (ITG) driven instability in toroidal plasma is derived from the reduced Braginskii equations. It includes the effects of both the safety factor q and the adiabatic electron temperature gradient . The present model reproduces the basic properties of ITG instability obtained by previous works. On the other hand, it is found numerically that the safety factor q enhances the ITG instabilities while the quantity reduces them .