Grain length is one of the components of rice grain shape that directly affects grain yield. Besides, grain length is one of the important indicators of rice quality. In this study, the longgrain restorer line R204 containing the blast resistance gene Pigm1 was used for genetic analysis of grain length. Through mapbased cloning, the longgrain gene was identified and its application in breeding was analyzed through analyzing cross combinations. The results showed that: (1) the GL121 (grain length 121) locus is under the genetic control of a dominant Mendelian factor. (2) Through mapbased cloning, GL121 was initially located to a chromosomal region of about 4.5 Mb between Inde123 and Inde127 on chromosome 12 of rice. (3) Further genetic analysis delimits the GL121 locus to a chromosomal region of 900 kb between markers Inde10 and Inde16. (4) F₁ hybrids from the test crosses with ‘Dingyuan A’, ‘Qiyuan A’ and ‘Qingyuan A’ showed the longgrain phenotype of the R204 line, whereas F₁ hybrids from the test cross with ‘Liangxiang A’ showed the grain length phenotype of the ‘Liangxiang A’ that is longer than that of the R204 line. The research showed that the GL121 locus is under the genetic control of a dominant Mendelian factor, which may be a new gene controlling grain length. Moreover, this study provides a foundation for cloning, characterization and functional analysis which are essential for molecular breeding of the grain type gene GL121.