Two types of hydrogels were synthesized by X-Ray irradiation of Poly-vinyl alcohol (PVA) and by using of cross-linking agent N,N'-methylene-bis-acrylamide for the synthesis of branched polymer Dextran-graft-Polyacrylaamide (D-g-PAA). Thermal characteristics of the samples were measuredby TGA method to analyze the effect of polymer concentration (for PVA) and effect of amount of cross-linked agent (for D-g-PAA) on the structure of hydrogels. Thermal behavior showed that the hydrogel structure mainly dependson the concentration of the cross-linking agent than on the initial polymer concentration in the case of the radiation cross-linking.