  We obtain Lewenstein-Sanpera decomposition of iso-concurrence decomposable
states of $2\otimes 2$ quantum systems. It is shown that in these systems
average concurrence of the decomposition is equal to the concurrence of the
state and also it is equal to the amount of violation of positive partial
transpose criterion. It is also shown that the product states introduced by
Wootters in [W. K. Wootters, Phys. Rev. Lett. {\bf 80} 2245 (1998)] form the
best separable approximation ensemble for these states.
