Nucleation and growth of interstitial type dislocation loops were investigated by fusion neutron irradiation and fission neutron irradiations with improved control. The density of loops formed by both irradiations increased linearly with the fluence. The average size of loops also increased. The rate equation analysis showed that the loops were nucleated directly by the defect processes in a cascade zone, not by the reaction among the freely migrating point defects in the matrix. If the rate of loop formation is assumed to be directly proportional to the energy of a primary knock-on atom (PKA) produced by a neutron, the threshold energy of 80 keV and the rate of loop formation of 0.05 were obtained.