Abstract: An experiment was conducted to improve the nutrient content of tapioca by-products (onggok) through fermentation by using cellulolytic bacteria (Bacillus amyloliquefaciens) as inoculums. The experiment was determination of the optimum conditions (dosage of inoculums, fermentation length and temperature) for tapioca by-products (onggok) fermentation based on nutrient quality and quantity of these fermented products. The study was conducted in experimental methods, using the completely randomize design in factorial with 3 treatment were: 1) A factor (Dosage of inoculums: A1 = 2%, A2 = 6%, A3 = !0%), 2) B factor (Fermentation length: B1 = 3 days, B2 = 6 days, B3 = 9 days) and 3) C factor (Temperature: C1 = 30oC, C2 = 40oC, C3 = 50oC). Results of study showed that optimum conditions of the fermentation of tapioca by-products (onggok) was at 2% dosage of inoculums, 6 days of fermentation length and 40oC temperature. This conditions can decrease 32% of crude fiber and increase 360% of crude protein which made the nutritional value of the product based on dry-substance was 7.9% crude protein, 2.75% crude fat, 11.55% crude fiber, 0.26% calcium, 0.17% phosphor, 2190 Kcal/kg metabolic energy and 65.95% nitrogen retention.