The development and application of the small-grain rice sterile line Zhuo201S (Z201S) has demonstrated its potential for mechanized hybrid rice seed production, leading to significant cost reductions.  However, the molecular mechanism responsible for the small-grain size characteristic of Z201S remains unclear.  In this study, we conducted a genetic analysis using near-isogenic lines constructed from Z210S, a small-grain rice sterile line, and R2115, a normal-grain variety.  The results revealed that the small-grain trait in Z201S is governed by a single partially dominant gene which also enhances grain number.  Through mapping, we localized the causal gene to the short arm of chromosome 2, within a 113 kb physical region delimited by the molecular markers S2-4-1 and LB63.  Transgenic analysis and gene expression assays indicated LOC_Os02g14760 as the most likely candidate gene, suggesting that the small-grain size trait of Z201S is controlled by a novel locus that has not been previously identified.