Sintered ores with superior high-temperature reducibility can enhance indirect reduction at the shaft and permeability at the cohesive zone through improving the softening-melting behavior, resulting in stable operation of the blast furnace at low coke rates. As the depletion of high-grade ore deposits limits control of the chemical composition of sintered ores, small pores less than 10 µm in diameter in the microstructure of sintered ores were focused on to increase the high-temperature reducibility. Sintering conditions for increasing small pores in constant raw material conditions were examined. Furthermore, considering the heterogeneous structure of the sintered ores, reduction behaviors of relict ores and assimilated structures were estimated individually. The contribution of each structure to small pore formation during sintering and their influence on high-temperature reducibility was discussed. Sintering with a sharp temperature profile led to many small pores in the sintered ores by increasing the amount of relict ores with small pores even in constant raw material conditions. Both, for relict ores and assimilated structures, low-temperature reducibility was determined by the total porosity including large pores, whereas the Al₂O₃ content in gangue minerals, the <10 µm pore volume fraction, and the amount of gangue mineral influenced high-temperature reducibility. Assimilated structures involving granular hematite contained many small pores, compared with other types of assimilated structures. Results of plant trials for two different methods to increase small pores, in relict ores and in assimilated structures, revealed their potential for improving the high-temperature reducibility of sintered ores without controlling the chemical composition.