Abstract This study investigated, for the first time, the potential role of Astragalus polysaccharide (APS) and APS nanoparticles on testicular blood flow (TBF) and semen quality in Ossimi rams. Fifteen sexually mature Ossimi rams were allocated randomly into two treated groups that orally administered either nano APS (2 g/ram/day; n = 5) or APS (20 g/ram/day; n = 5) for four weeks and a control group (n = 5). The nano-emulsion was prepared by adding corn oil to the APS solution, sonicated, centrifuged at 20,000 rpm, then washed 3–4 times, and vacuum dried overnight at 40 °C. The antioxidant activity of APS and APS nano-emulsion was evaluated in vitro. Blood collection and ultrasonographic assessment of the testes and supratesticular arteries (STAs) were conducted immediately before treatment (W0) and once weekly for 6 successive weeks after APS and nano APS administration (W1-W6). Serum testosterone (T) and estradiol (E2) concentrations were determined by ELISA kits, while nitric oxide (NO) and total antioxidant capacity (TAC) were measured spectrophotometrically. Moreover, semen collection and evaluation of some sperm parameters were performed once a week. Results revealed decreases (P