We report on the detailed analysis of the mean-pairwise peculiar velocity profile in high-resolution cosmological N-body simulations ( N = 8.8 \times 10 ^ { 6 } particles in a sphere of 50 \sim 200 Mpc radius ) .
In particular we examine the validity and limitations of the stable condition which states that the mean physical separation of particle pairs is constant on small scales .
We find a significant time-variation ( irregular oscillatory behavior ) of the mean-pairwise peculiar velocity in nonlinear regimes .
We argue that this behavior is not due to any numerical artifact , but a natural consequence of the continuous merging processes in the hierarchical clustering universe .
While such a time-variation is significant in a relatively local patch of the universe , the global average over a huge spatial volume \buildrel > \over { \sim } ( 200Mpc ) ^ { 3 } does not reveal any systematic departure from the stable condition .
Thus we conclude that the mean-pairwise peculiar velocity is rather unstable statistics but still satisfies the stable condition when averaged over the cosmological volume .