To establish the effect of daily oral treatment with different levels of L-Carnitine (LC) for 3 mo on body weight, blood parameters and reproductive performance of Friesian bulls, total of 9 bulls (360±32.1 kg LBW and 20±1.4 mo of age) were assigned into 3 groups. All bulls were fed the same diet and kept under the same condition, but differed in LC treatment. Bulls were received oral dose of LC for 3 mo at levels of 0, 1 and 2 g/h/d in G1, G2 and G3, respectively. At the end of treatment, semen was collected from bulls twice a week for 12 wk and evaluated for ejaculate volume,sperm motility, sperm livability, sperm cell concentration and total sperm output. At the end of semen collection, blood samples were collected from all animals for determination of total protein (TP), albumin (AL), globulin (GL), urea-N, cholesterol (CHO), glucose and total lipids (TL) concentration and activity of AST and ALT in blood serum. Also, testosterone concentration was determined in blood serum after 1 and 3 mo of treatment.   Results show that bulls in G3 showed the heaviest weights and gain as compared to G1, but did not differ significantly from those in G2. In blood serum of bulls, concentration of TP and GL increased (P<0.05), while AL/GL ratio and concentration of CHO and TL reduced (P<0.05) in G2 and G3 as compared to G1. Concentration of AL, glucose and urea-N or activity of AST and ALT were not affected by LC treatment. All semen characteristics of bulls improved (P<0.05)in G2 and G3 as compared to G1, being better (P<0.05) in G3 than in G2 for most characteristics. Serum testosterone concentration was higher (P<0.05) in G2 and G3 than in G1. In conclusion, oral dose of LC at a level of 2 g/h/d for 3 mo had impact to achieve high quality semen to spread the use of artificial insemination with bulls of high fertility.