In the recent years composite hydrogels have gained considerable interest as a biomaterial vehicle for biomedi- cal applications. The purpose of this study is to prepare highly biocompatible Polyethylene glycol (PEG)/Gelatin composite hydrogels for Glucose Oxidase (GOD) immobilization and evaluate the enzyme activity. PEG/Gelatin composite hydrogels were prapered by one step simultaneous technique based on UV-initiated free radical pho- topolymerization. PEG/Gelatin composite hydrogels were prepared by mixing Polyethylene (glycol) diacrylate PEG (30% - 50%) and Gelatin (22.000, 24.000, 40.000, 50.000, 87.500 MW) in the precense of ethylene glycol di- methacrylate (EGDMA) as crosslinker and 2,2-dimethoxy-2-phenylacetophenone (DMPA) as photo initiator and also GOD (0.1 mg/ml) for enzyme immobilization. Enzyme activity studies revealed that GOD immobilized PEG/Gelatin composite hydrogels that contain 30% PEG-DA presented a better activity, lower Km and higher Vmax than GOD immobilized composite hydogels that contain 50% PEG-DA. According to these results composite hydrogels are thought to be adapted for different enzyme systems with the selection of the suitable ratio.