In current work, the effect of nanoparticle size in Cu-Y2O3 alloy on the microstructure response after He/D sequential irradiation is studied using a transmission electron microscope (TEM) and nanoindentation. The results indicated that the direct contribution of nanoparticle/matrix interface to defect evolution and irradiation hardening of ODS Cu is limited when the nanoparticle size is larger than 20 nm. The difference in the proportion of ∑3 grain boundary and small angle grain boundaries was observed with different nanoparticle sizes, which suggests that the indirect effects on the sink strength caused by changes in the nature of grain boundary cannot be ignored, especially for the large particles. Besides, the distribution of bubbles after sequential He/D irradiation was highly similar to that in single He irradiation.