The present study aimed to investigate the effect of fennel seeds by-product (FSB) at different levels in rabbit feeds. The experiment was carried out at privet farm in Giza, Egypt. Thirty-six unsexed White New Zealand rabbits breed aged 5-6 weeks with average weight of 682.85±3.75g were randomly assigned into four equal groups: the first group was basal diet (R₁ control), meanwhile, the other three experimental groups were received diets containing 20, 40 and 60% of FSB for (R ₂, R₃, and R₄), respectively. The feeding period was extended for 56 days. Results showed that the best apparent digestibilities of DM, OM, CP, CF, NFE digestibility were recorded with rabbits fed 40% FSB diet (R3) in comparison with the other tested control diets. All rabbits received diets containing FSB recorded higher (P<0.05) nutritive values expressed as TDN%, serum level of HDL and mean values of HDL-C/LDLC ratio were significant (P<0.05) increase in diets containing FSB diets compared to control, the highest significant values recorded with diet (40%FSB). However, DCP%; serum total protein; albumin and albumin: globulin ratio values were not significant differences between the experimental groups. There was significant decreased (P<0.05) for treated diets of serum (AST) and (ALT), TG, TC, LDL, TC/ HDL and VLDL-C compared to the control group. The lowest significant values recorded with diet 40% FSB. The caecal NH₃-N concentration was higher ratios of R3 followed R2 and finally R4, while, the TVFA,s was significant increased in R3 and R4 only compared to the control. Both body weight gain and average daily gain were significantly (P<0.05) improved (by 6.24%, 26.58% and 12.66%) and (by 6.22, 26.55 and 12.64%, in groups fed diets contained 20, 40 and 60% FSB, respectively, compared to those fed the control diet. Inclusion of 40 or 60% FSB in the rabbit diets led to significant (P<0.05) differences in hot carcass weights and dressing percentages compared to the 20% or control FSB groups. Incorporation FSB in rabbit diets significantly (P<0.05) increased the dressing percentage and decreased fat % of carcass boneless compared to control group. Increasing FSB% in the tested diets decreased FSB feed consumption by rabbits compared to control. No mortality was recorded in the rabbits fed diets incorporated 40 and 60% fennel seeds by-product (R3 and R4 diets), while rabbits fed diets containing 0 (control) and 20% FSB (R1 and R2 diets) were recorded 8.3% mortality rate. The major phytochemicals of the fennel seeds by-product are the total antioxidant capacity 183.82 mg/100g (ascorbic acid equivalent), the total phenols 113.96 mg/100g (garlic acid equivalent) and total flavonoid contents 67.10 mg/100 (catechin equivalents)  addition of FSB improved feed conversion ratio;  total cost, total revenue, net revenue, economic efficiency, relative economic efficiency and feed cost / kg LBW. Rabbits received R3 which content 40% FSB recorded the best total cost. It can be concluded that adding 40% fennel seeds by-product in rabbit diets improved their nutrient digestibility, nutritive values, final body weight, average daily gain, feed conversion ratio and decreased mortality rate, as well as realized the highest value of relative economic efficiency.