Katanin, a microtubule severing enzyme, consists of two subunits, the catalytic subunit P60 and the regulatory subunit P80. In several species, P80 functions in meiotic spindle organization, the flagella biogenesis, the neuronal development and the male gamete production. However, the P80 function in higher plants remains elusive. In this study, we found that there are three katanin P80 orthologs (OsKTN80a, OsKTN80b and OsKTN80c) in Oryza sativa. Overexpression of OsKTN80a caused the retarded root growth of rice seedling. Further investigation indicates that the retained root growth was caused by the repressed cell elongation in elongation zone and the stalled cytokinesis in division zone in root tip. The in vivo examination suggests that OsKTN80a acts as a microtubule stabilizer. We prove that OsKTN80a, possibly associated with OsKTN60, is involved in root growth via regulating the cell elongation and division.