Thermal behavior of the Bi-2223 phase in the Bi, Pb-Sr-Ca-Cu-O system, especially segregation and dissolution reaction of the "3321" phase which was identified as Sr_2.5Bi_0.5Pb₃Ca₂CuO₂, has been studied by XRD, SQUID and resistivity measurements. On annealing below 830°C in air, the "3321" phase precipitated from the 2223 crystals. When the 2223 phase was annealed at 750°C in O₂stream, the precipitation of the "3321 " phase increased. On annealing at 750°C in Ar stream, however, no precipitation of the "3321" phase was detected. By reheateing at 850°C even for 1min, the "3321" phase was almost dissolved into the mother 2223 crystals. When the 2223 phase was reheated at 750°C in Ar stream after annealing at 750°C in air, the "3321 " phase disappeared. On the contrary, the "3321" phase was stable on reheating at 870°C in O₂ stream. Therefore, it seems that the "3321" and the 2223 phase coexist in epuilibrium in a high oxygen partial pressure.