ICR and C57BL/6J mice have been widely used in several research fields. The reproductive toxicology parameters, such as fertilization rate, which may differ between the two strains, are well known. However, the details of the sperm quality parameters are not well known. To reveal these, we compared the sperm morphology of the two strains. Eosin-stained sperm smears from adult ICR and C57BL/6J mice were analyzed. We observed that 79.6 ± 1.2 and 49.5 ± 1.7% of ICR and C57BL/6J mice sperm, respectively, showed a normal form. Furthermore, abnormal sperm samples were classified into ten types based on their defective sites. The percentage of abnormal sperm with an amorphous head, bent head, no head, hairpin loop, short tail, and two tails in ICR mice was significantly lower than that in C57BL/6J mice. In contrast, the percentage of coil-tailed sperm in ICR mice was significantly higher than that in C57BL/6J mice. These results suggest that C57BL/6J mice have a limited ability to remove the cytoplasm during spermiation and ICR mice have fewer sperm abnormalities than C57BL/6J mice. The characteristics of male reproductive traits among mouse strains should be taken into consideration in sperm analysis, as the negligence of this could generate an increased potential for a misleading in toxicology evaluation.