The 10 m diameter aggregates made of clay, carbon and Fe2O3 were prepared to investigate the mechanism ofblack core formation. The specific gravity, absorption rate, percent of black core area, fracture strength, total Fe analysis,sintering methods. Small addition of Fe2O3 did not afect physical properties of the aggregates; however, the percent ofblack core area increased with increasing carbon contents and increasing sintering temperature. Specific gravity of theaggregates decreased and the water absorption ratio increased with increasing percent of black core area. The aggregatessintered at oxidation atmosphere showed clear border betwen shell and black core area. Hence, the agregates sintered atreduction atmosphere showed only black core area in the cross-section of the aggregates. The specific gravity of thecomparing other agregates sintered at diferent atmospheres. Adsorption rate increased with increasing carbon contents atal atmospheres. The fast sintered agregates showed lower specific gravity, higher absorption rate, and more black corearea than the normally sintered aggregates. It was turned out that the aggregates having more black core area showedhigher fracture strength than that of aggregates with no black core area. From the total Fe analysis, the concentration of Feand FeO was higher at black core area than at shel. Because the concentration of Fe2O3 in the shell was higher than othercore area.