Technical Abstract: Straighthead, a physiological disorder of rice (Oryza sativa L.), has been reported in numerous rice-growing countries for its severe impact on yield reduction. Genetic diversity, relatedness and structure of parental germplasm are important for breeders to design strategies in breeding programs. We analyzed 1,002 rice accessions that were sampled from the USDA core collection, evaluated for straighthead and genotyped with 72 molecular markers. Forty-two accessions were resistant to straighthead with no yield reduction under severe infestation. The resistant cultivars originated from fifteen countries in ten geographic regions worldwide. The accessions had great diversification indicated by fifteen phenotypic characteristics including all types of grain and cooking quality. The number of alleles averaged 5.9, and the 72 molecular markers were highly informative among the 42 resistant accessions. Twenty-six resistant accessions were identified as indica type and were clearly classified in two groups. Ten resistant cultivars were identified as temperate japonica and the other two were positioned between temperate and tropical japonica rice. Breeders worldwide could use these findings on phenotypic diversification and genetic relatedness among the straighthead resistant accessions in order to make germplasm choices for transferring straighthead resistance to their local cultivars, serving various needs on grain, cooking quality, and genetic classification in their programs.