The join CaMgSi2O6-CaFe^[3+]AlSiO6 was studied by the ordinary quenching method at 10^-9, 10^-10, and 10^-11 atm fO2. The solubility limit of CaFe^[3+]AlSiO6 in diopside is 20 mol.% at 1100℃ and 10^-9 atm fO2, 10 mol.% at 1100℃ and 10^-10 atm fO2 and 5 mol.% at 1100℃ and 10^-11 atm fO2. The temperature of upper limit for clinopyroxene(ss) single phase field decrease with decreasing fO2. The phase assemblages below the solidus temperatures in this join change as follows with increasing CaFe^[3+]AlSiO6: clinopyroxene(ss), clinopyroxene(ss) + melilite, clinopyroxene(ss) + melilite + spinel(ss), clinopyroxene(ss) + melilite + spinel(ss) + anorthite. The composition of clinopyroxene(ss) crystallized from the composition Di(50)FATs(50) at 1100℃, 10^-10 atm fO2 is Ca0.96 (Mg0.48Fe0.12^[2+]Fe0.26^[3+]Al0.14) (Al0.44Si1.56)O6
