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In electric discharge machining, it is wellknown that any molten part remains as a melting remnant layer and under it a thermal affected layer is formed. This paper deals with an estimation of those layers based on the microstructure surrounding the impulsed discharge crater. The boundary between the melting remnant and the thermal affected layer can be clear. Furthermore, Acl and Ac3 transformation due to the higher heating and cooling action in an extremely short time are observed on the thermal affected layer. The Acl and Ac3 transformation temperatures are about 800C and 900C, respectively. The results present several thermal data for confirming the validity of a thermal conductive model for theoretical analysis of the size and form of the crater.