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Microstructure and thermal expansion coefficient of LaFe11.2Si1.8 alloy in different annealing processes were investigated by X-ray diffraction, scanning electron microscope and thermo-dilatometer. The results show that the sample annealed at 1200 oC for 5 h contains a certain amount of α-Fe phase, while the samples annealed at 1050 oC for 15 d and at 1100 oC for 2 d form a single La(Fe,Si)13 phase with the NaZn13 structure. The magnitude of the negative thermal expansion is linear with the La(Fe,Si)13 phase content. Typically, the magnitude of the negative thermal expansion and thermal expansion coefficient of the samples annealed at 1050 oC for 15 d are 3.22×10-3 and –39.31×10-6 K-1, respectively.