  We study the stability of NPT property of an arbitrary pure entangled state
under the mixture of arbitrary pure separable states. For bipartite pure states
with Schmidt number $n$ $(n>1)$ which is NPT, we show that this state is still
NPT when it is mixed with no more than $\frac{n(n-1)}{2}-1$ arbitrary pure
separable states. This result is generalized to multipartite case.
