Abstract: The addition reaction of Ni (Ⅱ)-Iminodiacetic acid (IMDA) with HBG and the displacement reaction of Ni(Ⅱ)-Tartaric acid (Tart) with HBG as well as the complexation of Ni(Ⅱ) with HBGwere studied by pH--metrically in aqueous solution at 30.0±0.1℃in the presence of 0.1M potassium chloride.The equilibrium constants of addition reaction and displacement reaction (k₁₁₁and k_r1),the over-all formation constants of two mixed-ligand complexes (β₁₁₁) as well as the step-wise formation constants of Ni(Ⅱ)-HBGchelate (k₁₀₁,k₁₀₂) and the over-all formation constant of Ni(Ⅱ)-Tart chelate (β₁₂₀) were determined.The results are summarized as follows;Ni(Ⅱ)-IMDA-HBG.lgk₁₁₁7.10,lgβ₁₁₁15.36;Ni(Ⅱ)-Tart-HBG:lg k_r15.13,lgβ₁₁₁ 12.68,lgβ₁₂₀7.55;Ni(Ⅱ)-HBG: lgk₁₀₁9.07,lgk₁₀₂5.54,lgβ₁₂₀14.61.The pHtitration curves,the stability of mixed-ligand complexes and Ni(Ⅱ)-HBGchelate as well as the possible chelated-form of Ni(Ⅱ)-Tart chelate in basic condition were discussed.