Abstract: 【Objective】 The purpose of this experiment was to investigate the effect of guanidine acetic acid （GAA） supplementation on the concentration of guanidinacetic acid and creatine in plasma of Yili horse. 【Method】 Twenty Yili horses aged 2 to 3 years old， similar in weight （373.2±30.53） kg， and rigorously trained for speed races， were randomly divided into 4 groups and with 5 horses （3♀+2♂） in each group， and the training intensity were consistent. Each horse was fed 4 kg of concentrate supplements per day， and freely collected dry herbage. On this basis， each horse in each group was fed with 0 （CON group）， 25 （25 GAA group）， 50 （50 GAA group） and 75 mg/kg BW （75 GAA group） guanidine acetic acid respectively. 【Result】 The results showed as follows：（1） Supplementation of guanidine acetic acid significantly increased the plasma Cr/GAA， of horses in 50 GAA group from 1 to 10 days and 75 GAA group for 7 days and 10 days at 2 hours after feeding （P<0.01） ， and significantly increased the plasma creatinine concentration at 2 hours after feeding for 7 days and 10 days in 50 GAA group （P<0.05）. （2） Supplementation of guanidine acetic acid significantly increased plasma Cr/GAA in all experimental groups （P<0.01）， and plasma creatine concentration and 24 h urine creatine metabolite excretion in 50 and 75 GAA groups were significantly increased （P<0.01）. 【Conclusion】 It can be concluded that under the conditions of this experiment， guanidine acetic acid supplementation increased the concentration of plasma creatine metabolites and promoted the rapid absorption of guanidine acetic acid in Yili horse， and the suitable level of guanidine acetic acid supplementation was 50 mg/kg BW.