Sensitive and selective detection for cancer biomarkers is critical in cancer clinical diagnostics. In this work, we report an electrochemical detection platform for the carbohydrate antigen tumor marker 15-3 (CA15-3). It is based on a composite material of poly [2-methoxy-5- (2-ethylhexyloxy) -1,4-phenylenevinylene] (MEH-PPV) and multi-walled carbon nanotubes (MWCNT), which deposited as active layer of an organic thin-film transistor. This layer was surface functionalized with the Anti-CA15-3 monoclonal antibody. The devices have a favorable electrical output response for VSD source-drain potentials between 0 to 5 volts, and VG as 8 volts. Once the antigen (CA15-3) is recognized by the antibody, the electrical response is diminished. The test has a linear response in the concentration range of 0–30 U mL - 1 of CA 15–3, with a lower detection limit of ~1 U mL - 1 and a stability of 90% with respect to the initial values after storing the device for two weeks. The method was successfully applied to the determination of CA15-3 in serum samples. Possibly, this used composite material has a greater scope and can be applied to another type of detection scheme.