2,2',3,3',4,4',5,5',6,6'-Decabrominated diphenyl ether (PBDE 209) is the second most used brominated flame retardant (BFRs) in constructed materials because it is considered less toxic than others, though other fire retardants, some congeners of PBDE 209, are reported to be toxic. This combined the fact that PBDE 209 has been found in high levels in human milk, blood, indoor environments as well as in foodstuffs has led us in this study attempt to find out whether PBDE 209, also known as decaBDE and decabrominated diphenyl oxide (DBDPO), has an adverse effect on this histology of testes and sperm in CD-1 male mice. The mice we studied were divided into groups and gavaged with 10, 100, 500 and 1500 mg/kg PBDE 209 in corn oil per day between postnatal Days 21 and 70. On Day 71, the mice were anesthetized and sperm function, testis DNA content, and histopathology were studied. We found in the 500- and 1500-mg/kg/day groups that neonatal exposure to PBDE 209 reduced sperm epididymal sperm mitochondrial membrane potential (MMP), reduced amplitude of the lateral head displacement (ALH) and induced the generation of hydrogen peroxide (H2O2) in the sperm of sexually mature male mice, without affecting the sperm count, motility, morphology, curvilinear velocity (VCL), angular progressive velocity (VAP), straight-line velocity (VSL), beat-cross frequency (BCF), sperm chromatin structure assay (SCSA), superoxide anion (O2-*) generation, DNA content in testis cells, or testicular histopathology. ALH was positively associated with an increase in MMP and negatively associated with generation of sperm H2O2. The reduction of MMP was negatively associated with an increase in generation of sperm H2O2. The presence of the relationships between sperm ALH, MMP, and generation of H2O2 indicate toxic action possibly resulting from PBDE 209-induced oxidative stress. In conclusion, this is the first study to report the lowest-observed-adverse-effect level (LOAEL) for sperm function to be 500 mg/kg of PBDE 209 in male mice. Decreased epididymal sperm MMP and ALH as well as induced generation of sperm H2O2 were some of the most serious effects of postnatal PBDE 209 exposure. Future investigations should be performed to study the effects of prenatal exposure of PBDE 209 and the mechanism behind PBDE 209-related oxidative stress in the fetal and pubertal stages of development.