Abstract: Rice floral organ was the foundation of grain formation, and how to improve the yield of rice was the focus of study of rice genetics and breeding workers all the time. A clustered spikelets mutant (Cl) controlled by recessive gene was obtained from a M2 population of advanced backcross populations through EMS mutagenesis. The phenotype of the mutant showed that 2-4 spikelets clustered on the stem of panicle branch with a percentage of clustered spikelets up to 70% and the main axis degraded badly. A F2 population was developed from a cross between a japonica cultivar Nipponbare and the Cl mutant. Genetic analysis indicated that the trait was controlled by recessive gene but the ratio of normal plants to mutant plants in F2 population was deviated from the ratio 3:1. The gene Cl was mapped between SSR markers CGY-08 and CGY-02 on chromosome 4 of rice with a physical region of about 145.7kb by BSA method. Sequencing results of predicted genes in this region showed that there was a base change in exon 4 occurred in a P450 gene named CYP724B1, which encoded proteins related to synthesis of brassinosteroid (BR). Therefore, according to the phenotype of the mutant and sequencing results, the authors speculated that the putative cytochrome P450 was a candidate gene of mutant EMS-Cl.