The hydroxides co-precipitation method is widely used in the treatment of wastewater containing dilute toxic anions, but detail mechanism is so far not well. This study discusses the co-precipitation removal of fluorine (F(-I)) or arsenate (As(V)) in wastewater with aluminum hydroxide by analysing filtrates and precipitates using artificial wastewater and compared experimental results between F(-I) and As(V) to discuss the co-precipitation mechanism. Analysing F(-I) and aluminum concentrations in filtrates showed aluminum hydroxide precipitation was hardly formed and F(-I) was hardly removed according to the complex formation between aluminum and F(-I) ions in the condition which F(-I) sufficiently existed much larger than aluminum. When the initial F(-I) concentration in wastewater was 30 mg dm-3, the residual F(-I) concentration was reduced less than 8 mg dm-3 (national effluent standard in Japan) by addition of 90 mg dm-3 aluminum at pH 5 and 60 mg dm-3 aluminum at pH 7. Zeta potential measurement or X-ray diffraction (XRD) patterns showed that removal of F(-I) was achieved by adsorption where aluminum hydroxide surface in the condition which aluminum hydroxide precipitation was formed when initial F/Al molar ratio was less than three whereas it was achieved by surface precipitation when initial F/As was more than three. Moreover, analysing precipitations by XRD showed crystalline K2AlF5 H2O was formed in bulk solution in the condition where large amount of both F(-I) and aluminum existed in solution and besides F/Al molar ratio was more than five. On the other hand, for As(V) case, aluminum hydroxide was well precipitated because any complex formed between As(V) and aluminum ions in the solution. When the initial As(V) concentration in wastewater was 10 mg dm-3, the residual As(V) concentration was reduced less than 0.1 mg dm-3 (national effluent standard in Japan) by addition of 40 mg dm-3 aluminum at pH 5 and 60 mg dm-3 aluminum at pH 7. Zeta potential measurement or XRD patterns showed removal of As(V) was achieved by adsorption on aluminum hydroxide surface when initial As/Al was less than 1.5 whereas it was achieved by surface precipitation when initial As/Al was more than 1.5.