The stability range of lead oxide compounds Pb3Sr2.5Bi0.5Ca2CuOy (3321) phase in Bi-system high temperature superconducting materials has been investigated. In this study (Bi,Pb)2Sr2Ca2Cu3Ox (BSCCO) powder was heat treated in different oxygen partial pressure and different temperature. The formation and the relative volume fraction of the 3321 phase were determined by X-ray diffraction. The microstructure of the quenched pellets was observed using SEM. Experimental results show that the stability range of the 3321 phase depends on both the temperature and oxygen partial pressure. A three dimensional plot of relative 3321 phase formation was obtained for BSCCO powder in which the main phase is (Bi,Pb)-2212 at different temperature and oxygen partial pressure.