We extend the theory of kinetic critical nucleus [K. Nishioka and I. L. Maksimov, J. Cryst. Growth 163, 1 (1996)] to binary systems for vapor-liquid transition and derive the equations to determine its size and composition. The kinetic critical nucleus corresponds to the extreme point (or saddle point) on the surface of the kinetic potential. By employing an available formula for the reversible work of forming a cluster, it is shown that the composition of the kinetic critical nucleus is the same as that of the thermodynamic one and the equation to determine the size of the kinetic critical nucleus has the same features as those for single-component systems. Similar to single-component systems, there are two values for the size of the kinetic critical nucleus.