Experimental study of the charge transport properties associated with structural variations due to a change in the ionic environment will provide essential physical information in determining the nature of DNA molecules. This work reports an experimental study of the change in electronic transport properties induced by the conformational transition of a poly d(GC)₄ DNA. By gradually increasing the concentration of MgCl₂ in the buffer solution from 0 M to 4 M, the conductance of the single DNA molecule decreased by two orders of magnitude. Circular dichroism (CD) measurements confirmed that a B to Z conformational transition caused the reduction in conductance. Using a stretch-hold mode scanning probe microscopy break junction (SPMBJ) technique, this B-Z transition process was monitored and a transition trend line was successfully achieved from conductance measurements alone. The transition midpoint occurred at a MgCl₂ concentration of 0.93 M for this DNA sequence. This method provides a general tool to study transitions of molecular properties associated with conductance differences.