This work investigated the effects of neutron irradiation on microstructure development in pure and dispersion-strengthened W (i.e., lanthanum (La)-doped W and potassium (K)-doped W). The irradiation experiments were performed in the experimental fast reactor Joyo. The irradiation temperature and damage were in the range 531-756 °C and 0.42-0.47 dpa, respectively. Voids, dislocation loops, and black dots were observed in pure, La-doped, and K-doped W after neutron irradiation with almost identical size and number density of the defect clusters. Likewise, the hardening of pure, La-doped, and K-doped W because of neutron irradiation was nearly identical. Thus, the dispersion of the second phase in La-doped and K-doped W did not affect the microstructure development and irradiation hardening in tungsten under neutron-irradiation conditions in this work.