We present magnetoresistance (MR) characteristics of 50 Angstrom Ta/40-80 Angstrom NiFe/22 Angstrom Cu/20 Angstrom Co90Fe10/150 Angstrom Ir20Mn80/(50) Angstrom Ta spin valves with temperatures ranging from 298 to 570 K in view of free layer NiFe thickness. The MR ratio ( = DeltaR/R) for a 40 Angstrom NiFe spin valve showed the highest value among the other thicker NiFe spin valves between 298 and 480 K. A nearly linear decrease in the MR ratio was observed between 298 and 380 K in all spin valves and the temperature coefficients of the MR ratio increased with decreasing NiFe thickness and the values were -0.0173, -0.0134, and -0.0111 %/K for 40, 60, and 80 Angstrom NiFe spin valves, respectively. The characteristic temperature T-OSV which was deduced to extrapolate to zero DeltaR/R were dependent on NiFe thickness and the T-OSV were determined to be 644, 690, and 639 K for 40, 60, and 80 A NiFe spin valves, respectively. Above 510 K, a steep decrease in DeltaR/R was found as a result of the combination of the free layer NiFe loop and the pinned layer CoFe loop due to the reduced CoFe/IrMn exchange coupling field. The exchange coupling field of CoFe/IrMn also showed the linear decrease with increasing temperature. The blocking temperature which were deduced to extrapolate to zero exchange coupling field were determined to be 519, 524, and 499 K for 40, 60, and 80 Angstrom NiFe spin valves, respectively.