To investigate the effect of dietary nano selenium (Se) on the level of Se in cow’s milk, ten healthy Holstein dairy cows were divided into two groups of 5 animals each, i.e., control and test groups. The control group was fed a basal diet and the test group was additionally given 15 g of dietary nano selenium each day for each cow. The results indicated that despite having no significant effect on the contents of milk fat, protein and lactose (P > 0.05), dietary nano selenium significantly increased the level of Se concentration compared with (P < 0.05). The concentration of milk Se was increased gradually with prolonged dietary supplementation, reaching 29.4 μg/L on day 30 compared to only 16.6 μg/L for the control group (P < 0.05). From these results it can be concluded that dietary supplementation with nano-Se could increase the concentration of Se in milk.