Naphthol Green B (NGB) is a synthetic azo dye widely used in various industries, including textiles and leathers. NGB poses significant environmental and ecological concerns when released into natural water systems. This paper investigates the decolorization of NGB using UV/sulfite system. The % decolorization of NGB was optimized using 3² Full Factorial Design (FFD), and the ANOVA results show that the model has a good fit for the data (R² = 99.54 %, R² _(adj) = 98.76 %) and the significant factors contributing to the % decolorization are A, B, A², and B² where A = mM sulfite and B = pH. The model predicted ≥100 % decolorization with the optimum conditions 12 mM sulfite and pH 10. An actual experiment was conducted to verify the response, resulting in 96.2 % decolorization which is in good agreement with the model.