By solving numerically the one-dimensional time-dependent Schrodinger equation, we have studied the effect of internuclear separation of diatomic molecular ion on ionization and high-order harmonic generation. The results reveal that when the internuclear seperation is small, both the ionization and the harmonic generation are weak, with the increasing of the separation, the ionization increases quickly, around some vaiue the ionization reaches a peak which is greater than what it is at small and large distances. Before the ionization reaches its maximum value, there is a reperation range where the harmonic generation is strongest, after the ionization reachees its peak value, the harmonic generation is decreased or eliminated. With the increasing of the separation further, the ionization becomes weak and the harmonic generation recovers to it's high level.