To investigate the effect of alloying elements on void formation and precipitation during neutron irradiation, vanadium and vanadium based alloys were irradiated in the experimental fast breeder reactor “JOYO” up to 6 × 10²⁵ n/m² at 670 to 870 K. Alloying elements were selected on the basis of volume size factors: carbon for an interstitial atom, chromium for an undersized substitutional atom and titanium and molybdenum are oversized ones. Voids were formed in all materials except in V-Ti alloys. The V-Cr alloy shows inhomogeneous void formation, and the swelling was larger compared to other alloys. The void formation was related to the radiation-induced precipitates which nucleated on dislocations. In the matrix of the V-Mo alloy, the precipitation was not confirmed, and homogeneous void formation occurred so the swelling was not large. Void formation and precipitation are discussed on the basis of alloying effects.