Direct electron transfer process of immobilized horseradish peroxidase (HRP) on a conducting polymer film, and its application as a biosensor for H 2 O 2 , were investigated by using electrochemical methods. The HRP was immobilized by covalent bonding between amino group of the HRP and carboxylic acid group of 5,2':5',2''-terthiophene-3'-carboxylic acid polymer (TCAP) which is present on a glassy carbon (GC). A pair of redox peaks attributed to the direct redox process of HRP immobilized on the biosensor electrode were observed at the HRP|TCAP|GC electrode in a 10 mM phosphate buffer solution (pH 7.4). The surface coverage of the HRP immobilized on TCAP|GC was about 1.2x10 - 1 2 mol cm - 2 and the electron transfer rate (ks) was determined to be 1.03 s - 1 . The HRP|TCAP|GC electrode acted as a sensor and displayed an excellent specific electrocatalytic response to the reduction of H 2 O 2 without the aid of an electron transfer mediator. The calibration range of H 2 O 2 was determined from 0.3-1.5 mM with a good linear relation.