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The genetic similarity and genetic difference among improved japonica rice varieties from different countries (or regions and organizations) were detected.The aim is to provide genetic basis to the breeding of japonica rice varieties.The genetic similarity and cluster of 313 improved japonica varieties from 20 countries (or regions and organizations) were analyzed using the SSR marker.With 34 SSR primers which were polymorphic and uniformly distributed in rice genome,totally 198 alleles were detected among these improved varieties with the average number of alleles per pair of primers of 5.8235.RM320,RM531,RM1,RM21,and RM336 located more alleles,which were 16,13,12,10,and 10 respectively.RM320,RM336,RM286,RM531,and RM21 showed higher genetic diversity indexes,which were 2.3668,2.0041,1.9684,1.9508,and 1.7203,respectively.The genetic similarity for improved japonica varieties among different countries (or regions and organizations) were ranged from 0.279 to 0.918,and the mean value was 0.653.The rice varieties from countries whose latitude and geography position were all nearer were clustered together with higher genetic similarity indexes.The rice varieties from countries who had more different latitude and far geography position were clustered separately with lower genetic similarity indexes.The results indicated the genetic similarity indexes among improved japonica varieties had a close relationship with the geographical position,especially with the latitude.