Iron mixed-valence complex (n-C3H7)(4)N[(FeFeIII)-Fe-II(dto)(3)] (dto=C2O2S2) exhibits not only the ferromagnetic transition at 6.5 K but also the charge-transfer phase transition between Fell and Fe-III around 120 K. In order to elucidate the mechanism of the charge-transfer phase transition, we measured the magnetic susceptibility under external pressure. Under hydrostatic pressure up to 0.9 Gpa, the critical temperature of the charge-transfer transition strongly depends on external pressure, while the Curie temperature remains unchanged. Under uniaxial stress of 0.6 GPa, on the other hands, the Curie temperature increases up to 15 K, while the charge-transfer temperature remains unchanged. After releasing uniaxial stress, the zero field magnetization at ambient pressure has two peaks at 7 K and 15 K, which implies two kinds of ferromagnetic spin configuration coexist, i.e. Fe-III (S=5/2), Fe-II(S=O) and Fe-III (S=1/2), Fe-II (S=2). The coexistence of two kinds of ferromagnetic spin configuration was also observed for (n-C4H9)(4)N[(FeFeIII)-Fe-II(dto)(3)] at ambient pressure.