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The strong thermal hysteresis of thermoelectric power (TEP) in Pr 0.5 (Sr, Ca) 0.5 MnO 3 is observed. A simple estimation shows that the electrostatic energy between Mn 3+ /Mn 4+ alone cannot account for the large shift of T co with changing A size. Based on our experimental evidence, we attribute the distinct features exhibited in the thermal hysteresis to the manifestation of the underlying competition between the delocalization and localization effects assisted respectively by the double exchange interaction and the charge ordering. The observed features include the following: (1) the A-size effect can depress T co by as much as 70 K for Pr 0.5 Sr 0.5−x Ca x MnO 3 ;(2) large thermal hysteresis of resistivity and TEP are exhibited in Pr 0.5 Sr 0.5−x Ca x MnO 3 ; (3) for x = 0.3 and 0.4, the thermal hysteresis is larger than those of x = 0.1 and 0.5 and (4) magnetic field induces a large shift in T co .