Owing to the effect of the transformation-induced plasticity (TRIP) or the twinning-induced plasticity (TWIP), Fe–Mn-based alloys have excellent mechanical properties and attract more attention as a promising structural material. Thin strip continuous casting was considered as an effective method to produce high-Mn steels, which were hard to produce by traditional methods. In this paper, the effect of different cooling rates by changing the trip thickness on the microstructure of Fe–15 wt-%Mn alloy prepared under sub-rapid solidification was investigated. The experimental results showed that α′-martensite and ϵ-martensite directly formed in the sub-rapidly solidified strips. The formation of ϵ-martensite in the 1·5 mm and 1 mm thickness strips suppressed with increasing cooling rate, while some nanocrystallines were obtained in these strips. The size of martensite phases was very fine in all strips, and the ϵ-martensite still existed after the heat treatment at 1000°C for 2 h.