An effective elongation method has been developed to study electronic structures and electron transport properties of nanoelectronic and bioelectronic devices at a hybrid density functional theory level. It enables to treat finite nanostructures consisting of as many as 28 000 electrons and has been successfully applied to sub-120-nm-long conjugated polymers, sub-60-nm-long single-walled carbon nanotubes, and 30 base-pair DNA molecules. The calculated current-voltage characteristics of different systems are found to be in good agreement with the experiments. Some unexpected behaviors of these nanosized devices have been discovered.