Thermal stabilization is used to reduce the turbidity of spent foundry sands (SFSs). Effect of stabilized temperature and thermal stabilized time in thermal stabilization is estimated by turbidity of SFSs in thermal stabilization. The turbidity of furan sand is 984 FAU and almost 15 times as high as that of other sands such as CO2 sand and green sand. Furan sand contains furan resin, but CO2 sand and green sand do not use resin chemicals. The turbidity of furan sand can be reduced by stabilization of furan resin in thermal process. In the process of thermal stabilization, fixation of furan resin by heating occurs and resin can be in the state of insolubility. The turbidity of furan sand is sharply decreased within thermally stabilized time of 2 hrs and decreased with increasing stabilized temperature. Turbidity in thermal stabilized time of 2hrs and stabilized temperature of 600°C is almost 10 FAU. Hence, thermal stabilization can be applied to reduce the turbidity of SFSs generated from iron foundry industry.