We prepared a new composite gel fiber by the gel formation of cellulose acetate (CA) and zirconium butoxide. Gel formation must be due to the coordination of ZrO2 to OH and CO groups on CA. Urease was entrap-immobilized on the fiber. The immobilization was easily performed under the mild conditions. The pH and thermal stabilities of immobilized urease were higher than those of the native one. The Michaelis constant (Km) and maximum reaction velocity (Vmax) were estimated from Hofstee plots for both native and immobilized urease. The Km of immobilized urease was larger than that of native urease, whereas the opposite tendency was observed for the Vmax. The Km for the immobilized urease became higher with increasing the fiber diameter. It indicates that the hydrolysis of urea occurs in the neighborhood of the fiber surface. Also, the immobilized urease gave a fairly stable property at 37°C for period up to 50 days.