In 1991, the Ministry of Agriculture, Forestry and Fisheries (MAFF) of Japan launched the Rice Genome Research Programme (RGP), with the aim of fully decoding the rice genome in three phases over a 21-year period. With the cooperation of 10 participating countries [2], the genome sequencing of 12 rice chromosomes was completed in 2005 [3]. Following this achievement, molecular genetic studies based on the results of the genome sequencing project became the most powerful tool for selecting mutants of certain characteristics in rice. This is anticipated to revolutionize mutation breeding success in rice, and become applicable to a number of other important crop species. In this report, the mutant cultivars developed mainly by Gamma-rays are discussed. In addition, their economic impacts in Japan, as well as molecular studies performed to elucidate the mutation at the DNA level are described.