Application of magnesium hydroxide as a coagulant for treating high pH reactive orange wastewater was studied. The coagulation performance and magnesium hydroxide-reactive orange floc properties were investigated under different dosages, feeding modes and pH values. Flocculation index (FI) was then discussed with controlled experiments using an intelligent particle dispersion analyzer and optimum coagulant dose of 150 mg/L (magnesium ion) was obtained for pH value 12. The results showed that the optimum magnesium ion dose tended to decrease with the increase of initial pH value. One time addition feeding mode led to relatively large FI value and higher removal efficiency compared with other addition modes. All of the flocs under investigation showed a limited capacity for re-growth when they had been previously broken. Based on the changes of zeta potential and floc properties, charge neutralization and precipitate enmeshment were proposed to be the main coagulation mechanisms.