A new and simple method has been proposed to prepare magnetic Fe(3)O(4)-chitosan (CS) nanoparticles by cross-linking with sodium tripolyphosphate (TPP), precipitation with NaOH and oxidation with O(2) in hydrochloric acid aqueous phase containing CS and Fe(OH)(2), and these magnetic CS nanoparticles were used to immobilize lipase. The effects on the sequence of adding NaOH and TPP, the reaction temperature, and the ratio of CS/Fe(OH)(2) were studied. TEM showed that the diameter of composite nanoparticles was about 80 nm, and that the magnetic Fe(3)O(4) nanoparticles with a diameter of 20 nm were evenly dispersed in the CS materials. Magnetic measurement revealed that the saturated magnetisation of the Fe(3)O(4)-CS nanoparticles could reach 35.54 emicro/g. The adsorption capacity of lipase onto nanoparticles could reach 129 mg/g; and the maximal enzyme activity was 20.02 micromol min(-1)mg(-1) (protein), and activity retention was as high as 55.6% at a certain loading amount.