Ion-acoustic solitary waves are investigated in a cold collisionless relativistic plasma. Electron inertia is also considered. The Sagdeev’s pseudopotential is determined in terms of u, the ion speed and depends on v, the velocity of the wave. It is found that there exists a critical value of u1 (≠ 0), the value of u at which (u′)2 = 0, beyond which the solitary waves cease to exist. The critical value also depends on the relativistic parameter u0/c when u0 is the drift velocity of the ion and c is the speed of light.