Summary: Quantum dense coding properties between two identical and spatially separated atoms in free space with different initial states is investigated. It shows that dense coding capacity \(\chi\) experienced a sharp decline firstly and then gradually increased to be one steady value 1 with increasing \(\Gamma t\). The realization of dense coding capacity \(\chi\) is found to be strongly dependent on the initial states. It is worth noting that the initial pure state |\(ee \rangle\) is not useful for dense coding in this system, due to the dense coding capacity \(\chi\) is always less than 1(a valid dense coding capacity satisfies \(\chi > 1\)). Otherwise, for the initial entangled state and mixed state, one can obtain the valid dense coding capacity, the results show that one threshold value of \(t_c\) is exists, and when \(t < t_c\) the dense coding capacity is valid. Tuning the atomic distance between the two atoms slightly broaden the valid dense coding region and improve the value of \(t_c\). Decreasing the purity \(a\) of initial states not only broaden the region but also prolong the effective time where one can carry out the valid dense coding successfully.