The brittle of Ir single crystal in nature has attracted more attention due to the fact that Ir is a FCC metal. In this paper, nanoindentation experiments were performed on the (100) and (110) planes of Ir single crystal. The results show that the dislocation activation volumes of Ir single crystal were calculated to be 1.09 ?3、1.23 ?3 for the (100) and (110) planes based on the first pop-in data from load-displacement curves. These results clearly demonstrate that the formation of dislocation is probably nucleated heterogeneously from point-like defects, while for the homogeneous nucleation, the activation energy should reach 60.57eV, and the activation radius reaches 1.971nm. In the plastic deformation of Ir single crystal, the deformation dislocation density was found to be the order of 1014m-2, which agrees with the previously reported data, implying that the anomalously high dislocation density does not exist in this nanoindentation experiment. The brittle fracture of Ir in nature may be attributed to the extremely small dislocation activation volume, leading to a large number of dislocation sources, and eventually caused by severely dislocation interaction.