Aerated lagoon is a kind of the aerobic suspended growth process that can be used to treat domestic and industrial wastewater. Due to its considerable design flexibility, the aerated lagoon has gained wide applications in both developed and developing countries. This study aims to investigate the performance of aerated lagoons in the wastewater treatment plant in Bou-Ali industrial zone in Hamedan over a period of seven months from May to November 2004. Quality indexes such as pH, TSS, BOD₅ ,COD, total coliform and fecal coliform, Cu, Pb, Zn, total Cr in plant influent and effluent were determined. The average values of pH, TSS, BOD₅ ,COD, total coliform and fecal coliform, Cu, Pb, Zn, total Cr in plant effluent were measured to be 7.46, 84mg/l, 76mg/l, 176mg/l, 1.98´10⁴MP N /100ml, 3.31´10³MPN/100ml, 0.04mg/l, 0.13mg/l, 0.88mg/l, and 0.03mg/l, respectively. The removal efficiencies recorded for the contaminants studied were 73.56%, 91.58%, 89.95%, 99.968%, 99.713%, 62.24 %, 59.08%, 56.51%, 62.57%, respectively. Total and fecal coliform contents were greater than standard limits for irrigation recommended by Iranian Environmental Protection Organization while the rest were below the standard values. If the effluent from this plant should be reused for agriculture or landscape irrigation, reduction of microbial pollution to standard limits recommended by Iranian EPO or to guidelines set by WHO must be considered.