Roots are important organs in all vascular plants. They anchor the plant, and absorb water and nutrients from the soil. During the growth of the plant, both primary root elongation and root hair formation occur. In the present study, we aimed to clarify the effects of root hair formation on primary root elongation. We measured primary root length in several mutant plants with different number of root hairs. The mutants with many root hairs showed similar or somewhat inhibited growth of the primary roots as compared with that of the wild-type plants. However, the mutant with few root hairs showed significantly enhanced primary root growth as compared with that in the wild type plants. Our results suggest that root hair formation might inhibit primary root growth. As the number of root hairs decreases, the growth of primary root might increase.