Label-free and real-time detections of mismatched dsDNAs are demonstrated using MutS-protein-immobilized, single-walled carbon nanotube field effect transistor (SWNT-FET) devices. The E. coli MutS proteins specifically recognizing mismatched dsDNAs are immobilized on SWNT-FET devices that have been fabricated for high sensitivity using a shadow mask lithographic technique to obtain a thin and wide Schottky contact region. The MutS-immobilized SWNT-FETs have successfully detected 40 base pair dsDNAs having single G-T mismatches at the 20th base pair positions by displaying significant electrical conductance drops at as low as 100 pM concentration. Systematic control experiments have revealed that the signal changes indeed originated from specific recognitions of mismatched DNAs by the immobilized MutS proteins.