This research was conducted to study the effect of Achillea millefolium flower extracts on sperm functions of male albino mice both in vivo and in vitro. Two extracts that are the aqueous and the ethanolic were prepared from the flowers of Achillea millefolium, and then chemical detection was done to identify the active compounds present in each extract. The detected compounds were alkaloids, glycosides, flavonoids, tannins, resins, coumarins, terpenes and saponins. In vivo effect of the aqueous and ethanolic extracts included assays on the body weight, testis weight and some sperm functions including: motility, concentration, viability and percentage of morphologically abnormal sperms. These two extracts were administered at a dose of 200mg/kg/day, given intraperitoneally for 20 days. The results showed a decrease in sperm's motility and concentration, and an increase in the percentage of dead and morphological abnormality of sperm, but there was no change body and testes weight. In-vitro study was carried out on the effect of aqueous and ethanolic extracts on some sperm functions including sperm motility, viability and morphologically abnormal sperms' percentage both after one minute, and 30 minutes of incubation. Each of these two extracts was added at a concentration of 200µg/ml to the sperm suspension. After one minute, the results showed a decrease in sperm motility and an increase in the percentage of dead and morphologically abnormal sperms. While after 30 minutes of incubation, further decrease in sperm motility, and higher increase in sperms' dead and morphologically abnormal percentage were observed. For both in vivo and in vitro studies, there was a significant difference between the aqueous and ethanolic extract in which the aqueous extract was more effective on sperm functions than the ethanolic extract, and so the best is the aqueous extract.