Criterion between permanent coalescence and separation in head-on binary droplet collision is investigated experimentally and numerically. Colliding droplets are transformed into a torus-like disk that contracts to form an elongated droplet having rounded ends and a slim connecting neck. The results of numerical simulation are in good agreement with experimental results. The surface energy at the maximum elongation indicates the criterion for the separation only in the collision of equal-sized droplets. The nondimensional axial length at the criterion between permanent coalescence and separation is independent of the radius ratio of the droplets. The separation mechanism is discussed based on the fluid flows at the maximum elongation. In separation collision, the pressure at the connecting neck of the elongated droplet rises because the connecting neck of the droplet becomes slender. While the length of the elongated droplet decreases after the maximum elongation, the fluid flow stretching the connecting neck leads to the separation.