Abstract: The reaction of LiH with vapor in atmosphere and Ar was in-situ investigated by laser Raman spectroscopy and atomic force microscope (AFM), etc. Experiments show that a product layer composed of LiOH, LiOH•H₂O and Li₂CO₃ forms as a result of the immediate reactions of LiH with H₂O and CO₂. The reaction velocity differs at distinct locations. In detail, the product distribution in depth and the variation rules of Raman intensity were discussed. Growth rates of surface products at different time were measured by AFM. Owing to stress release, microcracks appear in growing process, redounding to continuous penetration of H₂O through the product layer into LiH bulk for further reactions. The reaction of LiH with H2O was considerably dependent on humidity that even in Ar with purity up to 99.995%, LiOH was still identified.