Morphology and accommodation of thin-plate martensites in an Fe-Ni-C alloy have been investigated with atomic force microscopy (AFM). The quantitative observation of surface relieves made it possible to determine the kinds of thin-plate martensite variants by combining with single-face trace analysis. The martensites keep complementary configuration of variants in order to make the overall strain minimize. Strain distribution induced by the martensite formation in the parent phase was calculated using the height of the surface relief obtained by AFM observation and it was found that a highly strained region exists near the front of the martensite.