The activity of polygalacturonase(PGase) produced by Penicillium citrinum was studied in solid state fermentation (SSF) using sugar beet pulp as substrate. A full factorial design was used to study the effect of 5 variables (yeast extract , pH , inoculum size, incubation period, and incubation temperature) on polygalacturonase production. Maximum polygalacturonase (PGase) production ( 132 U/g dry fermented substrate(U/gdfs) was obtained in 8 days at 30°C and pH 5.5 with yeast extract as the best nitrogen source (1.2%) inoculated with 1 ml of spore suspension (1.8×105 spores/ml). Using the optimized conditions of factorial design, maximum PGase production has been obtained by using 0.7 kGy of gamma radiation with an activity 152.2U/gdfs as compared to the parent strain (unirradiated). The enzyme was partially purified using 75% ammonium sulphate precipitation, dialysis, and gel filteration chromatography on sephadex G-100. The optimum pH and temperature of the enzyme were found to be 6.0 and 40°C respectively. The enzyme was found to be stable in the pH range 4-8 and showed high stability at temperature range 20-60°C. The effect of metal ions on enzyme activity was investigated. It was found that Mg+2, and Zn+2 stimulated PGase activity.