The effect of different nutrient composition media on the production of mannanase by Bacillus velezensis NRC-1 was investigated. Medium 1 (composed of (g/L), peptone, 2.0; (NH₄)₂SO₄, 1.5; MgSO₄·7H₂O, 0.5; K₂HPO₄, 10.0; locust bean gum, 10.0, urea and 0.3, pH 5.3) was found to be the most favorable medium for mannanase production which recorded, 2.19 U/mL, after 7 days of incubation at 30 ºC. Taguchi orthogonal array method was used for optimization of nutrient composition medium and culture condition for mannanase production. Eight nutrient factors were studied including peptone, ammonium sulphate, urea, magnesium sulphate, dihydrogen potassium phosphate, locust bean gum, pH and fermentation volume with three levels. Analysis of variance (ANOVA) revealed that peptone concentration, locust bean gum and pH of the medium were the most influencing factors with percent participation 19.25, 13.46 and 57.89%, respectively. An increase in mannanase production to 8.7 U/mL could be achived after the optimization process.