Effects of antiphase domain (APD) size and inter-twin spacing on the hardness of CuAu I-phase in the overaging stage were investigated by hardness testing, transmission electron microscopy, and X-ray powder diffraction. Overaging in CuAu I-phase upon progressive ordering at 300°C was found to proceed through three stages: stages I through III. In the stage I, twinning actively occurred, and most of the coherency strains were removed. Average inter-twin spacing slightly increased. These microstructural evolutions slightly decreased the hardness of the alloy. In the stage II, both APD size and average inter-twin spacing grew larger with time, leading to a continuous decrease in hardness. In the stage III, the APD size markedly grew larger, while the growth rate of average inter-twin spacing markedly slowed down. The continuous growth of APD size apparently contributed to the further decrease in hardness. Although the crystal structure changes during the CuAu I ordering, a perfectly ordered alloy with no planar defects was suggested to be not so strong as the corresponding disordered alloy.