This study was conducted to investigate the effects of fermented king oyster mushroom by-products diet (FMBD) on the growth performance, blood characteristics, and carcass traits of fattening pigs and its economics. The fermented diet mainly contained 40.0% king oyster mushroom (Pleurotus eryngii) by-products, 20.0% corn, 28.0% soybean meal, 0.1% supplemental probiotics and 0.08% cellulase. The mixed ingredients were fermented for 1 d at $37^{circ}C$ followed by 9 d at room temperature, after which they were dried at $45^{circ}C$ for 3 d. Pigs (n=96) were divided into eight heads per pen, four diet treatments and three replications. The basal diet (C) was substituted with 20% (T1), 50% (T2) and 80% (T3) FMBD. The concentration of crude protein (CP) was significantly higher (p<0.05) at the end of the fermentation period. The average daily gain (ADG) and feed efficiency were significantly lower (p<0.05) in T3 than C. Additionally, carcass grade was significantly better (p<0.05) in all treatments than C and the ratio of high carcass grade was higher. Although substitution of FMBD decreased growth performance, it improved carcass grade and decreased the feed cost of fattening pigs. Therefore, it was expected that the increase in the utilization ratio of FMBD will reduce the cost of animal production.