The smouldering combustibility of cellulose insulation treated with boric acid-borax-aluminium sulfate as combustion retardants are examined by cigarette ignition method and electrical cardrige heater method. The effectiveness of Aluminium sulfate as a third combustion are acceptable both smouldering resistance and flame resistance at 18% level of all examined formulation. As the proportion of Aluminium sulfate in the formulation was increased, the flame resistance of cellulose insulation was improved.