ABSTRACT This study aimed to evaluate the effects of different levels of synbiotic (Syn) on the growth performance, carcass traits, and serum parameters of Japanese quails. A total of 1,500 (750 male and 750 female) Japanese quails were assigned to five treatments, including the control (basal diet) and basal diets with the addition of 0.1, 0.2, 0.3, and 0.4% Syn according to a completely randomized design. Each treatment in the male and female experiment groups consisted of five replicates, with 30 chicks for each replicate. The addition of Syn had no effect on body weight (BW), while the BW of female quails was higher than that of male quails at day 42. The addition of Syn reduced feed intake (FI) and improved the feed conversion ratio (FCR) (P<0.000). The daily body weight gain and FI were significantly higher in female than in male quails and Syn also improved the FCR in the overall experiment. The effect of the addition of Syn on carcass parameters (except for abdominal fat) was not determined. The addition of Syn decreased abdominal fat weight and percentage when compared with the control group. Male quails were found to have higher carcass yield, wing, neck, and other percentages than female quails. However, female quails were found to have higher breast, breast skin, abdominal fat, and visceral organ percentages. The addition of 0.1% Syn to quail ration reduced serum aspartate aminotransferase, high-density lipoprotein (HDL), total protien (TP), and albumin (ALB) levels at day 42 of the trial. The addition of Syn decreased the serum total cholesterol level. Gender was found to have an effect in that female quails were found to have higher total cholesterol, HDL, TP, and ALB levels than male quails. In both genders, the addition of Syn improved the BWG, FC, and FCR in the overall experiment, reduced abdominal fat, decreased the total cholesterol levels, improved the HDL levels, and increased the TP and ALB levels at day 42. The addition of 0.3 and/or 0.4% synbiotic has a positive effect on quails by improving performance and enhancing some serum lipids and protein parameters.