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Results presented in this paper contribute to investigation of the effect of the added Ca-Si modifier amount (αk) on the microstructure scales of granular γ-(Fe, Mn)3C eutectics such as the volume fraction (f) and diameter (d) in the austenite steel matrix composites (EAMC).Directional solidification of EAMC has been carried out using vertical Bridgman method at 50.6μ ms-1 with a constant temperature gradient about 800Kcm-1 The higher constitutional supercooling ahead of solid-liquid interface attributing to the larger αk results in the enlargement of γ-(Fe,Mn)3C coupled-zone and the increment of the nucleation rate of eutectics. Therefore, f increases with increasing αk . The branches of the primary austenite dendrites develop more greatly as αk increases, which limits the growth of eutectics. As a result, d decreases with increasing αk.