The reductive perturbation method is employed to describe the behaviour of ion-acoustic waves for plasmas in the absence of magnetic field, leading to a type of modified Kadomtsev-Petviashvili equation. The stability of a special type of solitary wave solutions for the modified Kadomtsev-Petviashvili equation is investigated with a finite difference scheme. The numerical results show that this solitary wave is unstable under two particular initial perturbations.