The Erlonghu site represents the most northern city of the Yan State during the Warring States period. Metallographic study on the iron objects unearthed from the Erlonghu site reveals technological information about the types of iron at the site. Various iron objects tested include a Jue(similar to the axe), belt buckle, sickle, knife, chisel, pickaxe, sword and ring nail. They are all shaped by casting.The iron types studied include mottled cast iron, white cast iron, grey cast iron, ductile cast iron(acquired by decarburization annealing after casting), as well as spheroidal graphite particles. This work deepens the knowledge of the properties of metal tools, connotations of metal culture, iron and steel technology, as well as the relationships between the northeast and the central plains regions during the Warring States period.