Nowadays,industrial activities are so vigorous that litres of pollutants are generated every day from factories.Inside the wastewater,there are various kinds of heavy metals and chemicals which are toxic to both marine organisms and human body.Removal of these heavy metals before discharging wastewater into main streams has become a key study area in wastewater treatment.Activated sludge process is one of the secondary biological treatment in wastewater treatment plant.It makes use of the metabolic reactions of microorganisms to remove the nutrients and organic matters from the wastewater to ensure that the discharge will not pollute the marine environment.Inside different reactions of the activated sludge,nitrification is the main focus of this project.This project studied the oxidation rate of ammonium to nitrate in wastewater under the influence of heavy metals ions.Experiment on local activated sludge was chosen as the methodology of this study.Nitrification rate of activated sludge depends on different environmental factors,while concentrations and species of inhibitors are the dominant factors.In this project,investigations were done on the toxicity and inhibitory effects of the inhibitors to the nitrification process.Three heavy metals,cadmium,zinc and nickel,were chosen as inhibitors.Effective Concentration 50 (EC50) was used as classifying the inhibitory effect of three inhibitors.EC50 for cadmium,zinc and nickel are 4.78 mg/L,14.82 mg/L and 1.92 mg/L respectively.Comparison was also done among the toxicity of the three inhibitors.A toxicity sequence based on ECs0 was arranged in descending order of Nickel > Cadmium >Zinc.