Exchange anisotropy of Mn73Ir27/Co 70Fe30 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/cm 2 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 J K and increasing peak intensity of superlattice diffraction lines, the Mn3Ir phase was suggested to be an origin of the giant J K and the high blocking temperature.