An electrochemical aptasensor with a thrombin binding aptamer (TBA) was developed using a single-wall carbon nanotube (SWCNT) casted GCE. The TBA was immobilized on SWCNTs through -stacking without any special modification, resulting in helical wrapping to the surface. In the presence of thrombin, the TBA binds with thrombin and the TBA concentration on the SWCNT surface decreases. The remaining amount of TBA can be analyzed by an electrochemical method without any label, because the guanine bases of the nucleic acid are measurable by electrochemical methods. The electrochemical oxidation of guanine nucleotides was enhanced by electrocatalytic mediation using Ru(bpy)(3)(2+) for higher sensitivity and reduction of the overpotential for electrochemical detection.