Exchange anisotropy of Mn73Ir27/Co70Fe30 bilayers fabricated on a 50-nm-thick Cu under layer by changing the substrate temperature (Tsub) during the deposition of Mn–Ir layer was investigated, correlating with the crystallographic structure of Mn–Ir layer. The unidirectional anisotropy constant (JK) of the bilayers remarkably varied as a function of Tsub. After the thermal annealing of bilayers at 320 °C in a magnetic field of 1 kOe, JK steeply increased from 0.3 to 1.3 erg/cm2, as Tsub was raised from room temperature to 170 °C. The blocking temperature was enhanced from 270 to 360 °C, simultaneously. The JK of 1.3 erg/cm2 is nearly ten times larger than the values reported in Mn–Ir/Co–Fe bilayers early in the research of them. The x-ray diffraction profiles showed that the ordered Mn3Ir phase was formed in the antiferromagnetic layer with increasing Tsub. From the coincidence of enhancing JK and increasing peak intensity of superlattice diffraction lines, the Mn3Ir phase was suggested to be an origin of the giant JK and the high blocking temperature.