In metallography the shape of high carbon martensite is commonly considered to be ens–like in three dimensions, which is of needle or bamboo leaf on the OM micrographs of section lanes. However, a round martensite on OM micrographs have never been seen so far. This question as not been answered over the years. It has not been theoretically explanined why is martensite ath–like for low carbon steels and lens–like for high carbon steels on OM micrographs. For answering hese questions, the martensitic morphology of a steel with carbon content 1.37% was observed in the ame view by OM using a method of multiple sectioning of one sample. The length, width of the aimed artensite and sectioned thickness of samples were measured before observing. It is found that the artensite is a flat ellipsoid rather than lens–like as described in traditional textbooks. The ratio of a/b is about 3 and a/c is abot 20 for the martensite ellisoid. The thermal dynamic analyss ndicates hat the nucleation energy for a martenste s closely related to its shape. Ithe austenite with the ame grain size, the nucleation energy (ΔG^∗) of martensite will be lower with ellipsoid shape than with lens shape. The drvng force for martensitic transformation will reduce with the decrease of carbon ontent, resulting n the formation of lath–like martensite in steels with low carbon content.