  The time evolution of a maximally entangled bipartite systems is presented in
this paper. The distillability criterion is given in terms of Kraus operators.
Using the criterion, we discuss the distillability of $2\times 2$ and $n\times
n (n>2)$ systems in their evolution process. There are two distinguished
processes, dissipation and decoherence, which may destroy the distillability.
We discuss the effects of those processes on distillability in details.
