This article deals with some biological factors affecting the competition of nndulation between different strains of Rh. leguminosarum on vetch. The results are summarized as follows: 1. Nodulation competition of Rhizobium is mainly affected by the host plant. It is shown that host plants are quite susceptible to definite strains of Rhizobium, either in the case of a definite strain infecting different species o.f plant or in the case of various strains infPeting the same species of host plant. And, the susceptibilities of the Rhizobium to different species of host plant are more sensitive than that to the varieties of host plant.2. Rate of nodulation of the inoculant is decreased with the increase of the amount of native strains in soil. 3. Rate of nodulation is increased with the increase of the amount of inoculant applied in soil where native strains are populated. But, it is not true by seed inoculalion under the same soil condition.