Effect of Fe substitution for Cu(up to 3at%) on the high temperature phase equilibria of YBa₂ Cu₃ O_y was investigated under various oxygen partial pressures by electrical conductivity, thermoelectric power measurements and X-ray diffraction. The YBa₂ (Cu_1-x Fe_x )₃ O_y (x=0-0.03) had orthorhombic structure at room temperature. The electrical conductivity of YBa₂ (Cu_1-x Fe_x )₃ O_y (x=0-0.03) increased abruptly at about 450°C and the temperature dependence of the resistivity showed a slope change at the temperature which corresponds to the orthorhombic-tetragonal transition. Arrhenius plots of the electrical conductivities of YBa₂ (Cu_1-x Fe_x )₃ O_y measured under various Po₂ consisted of five straight lines. The temperatures at which the straight lines intersected were confirmed to correspond to the phase transition temperatures, which markedly decreased with increasing Fe content(x). The thermoelectric power of YBa₂ (Cu_1-x Fe_x )₃ O_y (x=0-0.03) is negative across the entire temperature range and Po₂ range investigated, which indicates that YBa₂ - (Cu_1-x Fe_x )₃ O_y (x=0-0.03) is of n-type. The temperature dependence of the thermoelectric power was very similar to that of the electrical resistivity.