Summary Physicochemical properties of storage starch largely determine rice grain quality and food characteristics. Therefore, modification of starch property is effective to fine‐tune cooked rice textures. To obtain new resources with modified starch property as breeding materials, we screened a mutant population of a japonica cultivar Nipponbare and found two independent mutant lines, altered gelatinization ( age ) 1 and age2 , with moderate changes in starch gelatinization property. A combination of conventional genetic analyses and the latest mapping method, MutMapPlus, revealed that both of these lines harbour novel independent mutant alleles of starch branching enzyme II b ( BEII b ) gene. In age1 , amino acid substitution of Met‐723 to Lys completely abolished BEII b enzyme activity without significant reduction in its protein level. A transposon insertion in an intron of BEII b gene reduced BEII b protein level and activity in age2 . Production of a series of the mutant lines by combining age alleles and indica ‐type starch synthase II a allele established stepwise alteration of the physicochemical properties of starch including apparent amylose content, thermal property, digestibility by α‐amylase and branched structures of amylopectin. Consistent with the alteration of starch properties, the results of a sensory evaluation test demonstrated that warm cooked rice of the mutants showed a variety of textures without marked reduction in overall palatability. These results suggest that a series of the mutant lines are capable of manipulation of cooked rice textures.