5-(Phenylethyl)-2'-deoxyuridine has been incorporated into an oligodeoxynucleotide (ODN) by using normal cyanoethyl phosphoramidite chemistry on a DNA synthesizer. For introduction of the modified residue at the 3'-end position of the ODN, we designed and synthesized a new nucleoside phosphoramidite derivative, which connected the 3'-hydroxyl group and phosphoramidite moiety by an alkaline-labile linker. The 3'-end could be substituted in ODNs by using commercially available supports as a starting material following standard NH4OH treatment. The ODN carrying 5-(phenylethyl)-2'-deoxyuridine at the 3'-end position showed about 3-fold resistance to nucleolytic degradation in human plasma without precluding its specific base-pairing activity.