The austenized transformation of carbon steels of 20, 45 and T10 as well as cast iron of HT200 has been investigated at some length with SEM analyses. We put stress upon studying the sites of austenitic nucleation and discussing the determination of transformation points, Acl and Ac3, of materials in laser heating. For Fe-C alloys, Ac1=A1 = 727 ℃, and for hypoeutectoid steels, Ac3=13603.9×(ln 0.22d/ξ-2v)-1.