Void initiation and growth serve as an important mechanism in ductile failures in metals. Particularly, on the micron-level, the extra hardening effect associated with strain gradient is accounted for by adopting strain gradient elasto-plasticity instead of the conventional plasticity. Effects of inertial, strain gradient hardening and thermal softening are formulated analytically for the case where a spherical void expands under external hydrostatic stress. As demonstrated by our results, the inertia effect firstly tends to hinder but then promotes the void growth. The threshold stress required for rapid void growth is lifted due to extra hardening of strain gradient so that the growth of a smaller void is delayed more remarkably. A considerable thermal softening phenomenon is observed here, which is caused by plastic work during the deformation process. The final void growth rate is mainly related to the maximum loading, which is consistent with the prediction based on the classical plastic theory.