Recent years have witnessed significant interest in biological applications of novel nanomaterials such as nanotubes with the motivation to create new types of analytical tools for life science and biotechnology. Single-walled carbon nanotubes (SWNTs) are interesting molecular wires with unique electronic properties that have been spotlighted for future solid-state nanoelectronics. Because semiconducting SWNTs could perform as nanoscale Schottky-type field-effect transistors, only semiconducting nanotubes exhibit a large conductance change. For the realization of nanotubebased electronic biosensors, it is necessary to manipulate metallic and semiconducting SWNTs separately. Unfortunately, the conventional synthesis approaches provide SWNT with mixed chiralities, which are not separable on a large scale with current technology. This has open been a bottleneck in the application of SWNT to biosensors that require ready-made semiconducting nanotubes. We have created label-free electronic DNA sensors via large-scale assembly of mixed carbon nanotubes without the need for separating. The electronic structure of metallic SWNTs can be modified by the coupling of π-electrons between a nanotube and aromatic molecules, and that SWNT-based sensors can be use for detecting biological molecules readily without the need for labeling. Sensing for label-free DNA detection was carried out by monitoring electrical current through the SWNT devices dominated by metallic property to each step of the π-stacking of pyrenly group, the immobilization of probe DNA, and the hybridization of target DNA. We observed that the conductance of SWNT film can be substantially decreased by π-stacking of pyrenly group, and regularly increased by probe DNA linkage and target DNA hybridization.