Urease (E.C 22.214.171.124) was covalently immobilized on activated methoxypolyethyleneglycol-5000 which is linear, uncharged, soluble in water and nonimmunogenic. mPEG is bound to the epsilon-NH2 groups of Lysin in urease. Previously different molar ratios of urease -Lys / activated-mPEG were searched for immobilization. Storage stabilities, molecular weights and the values of blocked amino groups were determined for each immobilized urease and the best conditions was found 1:3 urease-lys / activated mPEG. Furthermore physical characterization, kinetic constants (K-m, V-max), heat and temperature stabilites were also determined.