A boron doping technique was employed to determine the microstructural response to differences in the rate of helium generation in vanadium under fission and fusion irradiation environments. Helium is enriched in the regions around boride and somewhat in the matrix due to (n,α) reactions, when boron doped alloys are irradiated in a fission reactor. The approach is achieved by microscopy examination of the helium-rich regions adjacent to boride and those beyond the helium region, or by comparison of the microstructure in materials doped and undoped with boron. The specimens with the compositions of V, V-lat.%10B and V-0 6at.%10B-0.9at.%11B were prepared. The irradiations were performed at 673 K up to 0.75 × 1025 n/m2 (0.3 dpa) in the fast breeder experimental reactor JOYO. Helium generation by the transmutation of 10B(n,α)7Li was estimated in the present case to be mainly due to epithermal neutrons with the energy <100 eV. This paper describes the results of (TEM) observations following the irradiations. A high density of cavities were produced with smaller sizes near V3B2 precipitates, depending on the precipitate size and the distance from the precipitate surface. This was mainly attributed to the effects of lithium products around the precipitates, while helium products exhibited the tendency of reducing swelling slightly. The swelling in the matrix without the precipitates has been recognized to be lower in unalloyed vanadium than that in boron doped alloys, though the degree is rather small. It has been confirmed that helium reduces the number densities of cavities, accompanying the decrease of their sizes at the early stage of the irradiation (0.3 dpa) at 673 K. In addition, it was shown that the irradiation of vanadium induces the uniform precipitation of thin platelets with a lattice parameter of about 0.41 nm.