[Objectives]Starch is a major component of rice. The content,composition and fine structure of starch are crucial to the rice quality. Therefore,it is of great significance to dissect the molecular mechanisms underlying starch synthesis pathway and its regulation.[Methods]In this study,we obtained a stably inherited floury endosperm mutant flo9 from ‘N22’ mutant library induced by 60Co radiation,with slightly shrunken and homozygous lethal. The morphological characters and the physicochemical properties were analyzed. The endosperm structure was observed by scanning electron microscopy and semi-thin section technology. An F2 population of flo9 mutant and ‘Dianjingyou 1’ were prepared,and the floury individuals were selected for gene-mapping. The expression patterns of starch synthesis genes were analyzed by qRT-PCR.[Results]Compared to the wild type,the 1 000-grain weight,grain length,grain width,grain thickness,amylose content,and lipid content of the flo9 mutant all extremely decreased,while the total starch content had no change. The SBV(setback),CSV(consistance),BDV(breakdown)and the swelling power of starch of flo9 mutant were lower than the wild type. The starch grains in flo9 mutant significantly decreased in section area,were rounder and loosely packed. FLO9 was narrowed down to a 123 kb region on the long-arm of chromosome 9 with 314 recessive individuals. This region contains 13 open reading frames(ORFs). qRT-PCR analysis showed that the expression level of many genes related to starch synthesis and lipid synthesis were changed.[Conclusions]FLO9 was involved in starch and lipid biosynthesis in rice developing endosperm,as well as in amyloplast development. This study lays a foundation for the cloning and functional characterization of FLO9 in rice. [ABSTRACT FROM AUTHOR]