The linear stability of finite amplitude internal solitary wave propagating in a two- layer fluid is studied. We try to reveal the critical amplitude that divided the region of stability and instability in a parameter space of solitary waves. The set of equations construct eigen value problem. As a first step, We seek a numerical solution of these problems. As a second step we seek asymptotic solution by asymptotic analysis. Both analyses of them derived the stability criterion that can prescribe which sides of critical surface area stable or unstable.