Abstract: The E.coli L asparaginase was modified by N,O carboxymethyl chitosan. The modified enzyme remained about 70% catalytic activity. Experimental results also showed that the optimal pH of the modified enzyme was about 7.4 and its affinity of L asparagine was stronger than that of the native enzyme. It's stability against trypsin increased and the antigenity was only 1/3 of the native enzyme. It was found that the modified enzyme might be much more useful than that of the native enzyme in clinical treatment of tumor, especially for the acute lymphoblastic leukaemia.