We have developed an advanced electric probing system, which has two probes, with the spatial resolution of similar to1 nm and the detection limit of <1 pA in order to measure electric properties of nanometer-scale samples. This system consists of a conventional AFM system and a piezoactuator system. In electric measurements of samples, two probes must be connected to the sample with keeping an electric isolation between two probes. For a connection of probes with a nanometer-scale sample, the radiuses of curvature of the probes should be smaller than the sample size. Thus, we used carbon nanotube as one of two probes, so that we could measure current-voltage (I-V) curves of the nanometer-scale samples. We have applied our system to measuring I-V curves of a lambda phage DNA (lambda-DNA) bundle. The curves showed that the current through the lambda-DNA was less than similar to1 pA. In order to increase conductance of DNA molecule with chemical doping, we synthesized DNA-acceptor cross-linked derivatives (DACD). We measured I-V curves of DACD at various distances between two probes with our electric probing system. The results showed that the chemical doping increased the conductance of DNA. Furthermore, we explained the electrical transport mechanism through DACD with hopping model. (C) 2003 American Institute of Physics.