Improvement of the methods for oligonucleotide delivery into cells is necessary for the development of antisense therapy. In the present work, a new strategy for oligonucleotide delivery into cells was tested using cationic peptides as a vector. At first, to understand what structure of the peptide is required for binding with an oligonucleotide, several kinds of α‐helical and non‐α‐helical peptides containing cationic amino acids were employed. As a result, the amphiphilic α‐helix peptides were best for binding with the oligonucleotide, and the long chain length and large hydrophobic region in the amphiphilic structure of the peptide were necessary for the binding and forming of aggregates with the oligonucleotide. In the case of non‐α‐helical peptides, no significant binding ability was observed even if their chain lengths and number of cationic amino acid residues were equal to those of the α‐helical peptides. The remarkable ability of oligonucleotide delivery into COS‐7 cells was observed in the α‐helical peptides with a long chain length and large hydrophobic region in the amphiphilic structure, but was not observed in the non‐α‐helical peptides. It is considered that such α‐helical peptides could form optimum aggregates with the ODN for uptake into cells. Based on these results, the α‐helical peptide with a long chain length and large hydrophobic region is applicable as a vector for the delivery of oligonucleotides into cells. Copyright © 2000 European Peptide Society and John Wiley & Sons, Ltd.