  We characterize the separability of three qubit GHZ diagonal states in terms
of entries. This enables us to check separability of GHZ diagonal states
without decomposition into the sum of pure product states. In the course of
discussion, we show that the necessary criterion of G\"uhne for (full)
separability of three qubit GHZ diagonal states is sufficient with a simpler
formula. The main tool is to use entanglement witnesses which are tri-partite
Choi matrices of positive bi-linear maps.
