Agricultural utilization of wastewater sludge is practiced in many parts of the world. Sludge may contain a variety of pathogenic microorganisms, which can spread diseases if there is human exposure to the sludge. Therefore, sludge must be treated before disposal and reuse. In this study the effect of liming on the microbiological quality of urban liquid raw sludge in Isfahan Wastewater Treatment Plant (IWTP) was investigated. Sludge samples were taken from IWTP, and the initial concentrations of total solids, volatile solids, total coliform, fecal coliform, Salmonella and ova of parasites were determined. Then adding lime to increase the pH of sludge to 11 and 12 and the effect of liming on the parameters was carried out after 2, 24, 72 and 120 hours. Salmonella was inactivated completely in treated sludge after two hours. The removal of up to 99% of fecal coliform was obtained for two ranges of pH. However, reduction of ova of parasites at pH 11 and 12 after five days were found only 56% and 83.8% respectively. Sludge treated with lime meets USA Environmental Protection Agency (EPA) standards regarding Class B and Class A after 2 and 24 hours respectively. At pH higher than 11 and 12 treated sludge with lime meets vector attraction reduction requirements after two hours. However, at a pH higher than 11 and 12 treated sludge with lime does not meet the guideline for pathogen reduction of class a regarding eggs of parasites.