Abstract: The microstructurel evolution of micro-island-like sample during continuous temperature rise and fall (25-1100 ℃) was investigated by in situ observation.The results show that three processes are observed in the micro-island-like sample at the given heating and cooling process in accordance with block-like sample, including the decomposition of microstructure, nucleation and growth of austenite and martensitic transformation.The results indicate that grain boundaries, twin boundaries and phase boundaries can effectively provide nucleation sites of martensitic transformation and promote the martensitic transformation.Also, the boundaries can impede the growth of martensitic lath.In comparison with block-like sample, the martensitic transformation of micro-island-like sample follow an obvious sequence, and its priority depends on the position of connection with block-like sample.This demonstrates that the martensitic transformation is an inter-coordination and mutual promotion process among grains instead of transformation behavior in a single grain.In addition, the martensite has three substructures, including lath, block and packet.And no evident variant selection is found.