A catechol biosensor was developed and used to analyze compost extracts based on the immobilization of laccase on the surface of modified magnetic core-shall (Fe3O4-SiO2) nanoparticles. Laccase was convalently immobilized on the magnetic nanoparticles by glutaraldehyde, which were modified with amino groups on its surface. The resulting magnetic bio-nanoparticles were attached to the surface of carbon paste electrode with the help of a permanent magnet to determine catechol. The immobilization matrix provided a good microenvironment for retaining the bioactivity of laccase. The linear range for catechol determination was 7.5 x 10(-7)-2.75 x 10(-4) mol/L, with a detection limit of 7.5 x 10(-7) mol/L. The detection current reached 95% of the steady-state current within about 70 s. Catechol concentration in compost extracts were determined by laccase biosensor and HPLC, with approximately the same result.