Abstract: In order to explore the relationship between heat stress and nitric oxide synthase (NOS) and nitric oxide (NO) in the serum of Holstein and Jersey×Holstein dairy cows. Serum was collected from Holstein (n=20) and Jersey×Holstein F1 dairy cows (n=20) in heat stress (THI 82.61±0.60) and no heat stress period (THI 47.42±0.51). The levels of NOS and NO were measured. The result show that: the levels of NOS in serum in heat stress period were significant lower than those in no heat stress period both in Holstein and Jersey×Holstein F1 dairy cows (P<0.01). In heat stress period, levels of NOS of Jersey×Holstein F1 dairy cow were significant lower than those of Holstein dairy cow (P<0.01); but were no significant difference between Holstein and Jersey×Holstein F1 dairy cow (P>0.05) in no heat stress period. The levels of NO in serum in heat stress period were significantly lower than those in no heat stress period in Holstein dairy cow (P<0.05); but were no significant difference between heat stress period and no heat stress period (P>0.05) in Jersey×Holstein F1 dairy cow. In no heat stress period, the levels of NO of Holstein dairy cow in serum were higher than that of Jersey×Holstein F1 dairy cow (P<0.05), but in heat stress period there is no significant difference(P>0.05).