Tuberculosis is the most frequent cause of infection-related death worldwide. We constructed a simple and direct electrochemical sensor to detect interferon (IFN)-γ, a selective marker for tuberculosis pleurisy, using its RNA and DNA aptamers. IFN-γ was detected by its 5′-thiol-modified aptamer probe immobilized on the gold electrode. Interaction between IFN-γ and the aptamer was recorded using electrochemical impedance spectroscopy and quartz crystal microbalance (QCM) with high sensitivity. The RNA-aptamer-based sensor showed a low detection limit of 100fM, and the DNA-aptamer-based sensor detected IFN-γ to 1pM in sodium phosphate buffer. With QCM analysis, the aptamer immobilized on the electrode and IFN-γ bound to the aptamer probe was quantified. This QCM result shows that IFN-γ exists in multimeric forms to interact with the aptamers, and the RNA aptamer prefers the high multimeric state of IFN-γ. Such a preference may describe the low detection limit of the RNA aptamer shown by impedance analysis. In addition, IFN-γ was detected to 10pM by the DNA aptamer in fetal bovine serum, a mimicked biological system, which has similar components to pleural fluid.