Disclosed is an oligonucleotide-modified nucleic acid containing at least one 1-ß-D-arabinofuranosylcytosine as a modified nucleic acid having therapeutic efficacies and guanosine. More particularly, a novel oligonucleotide-modified nucleic acid containing at least one modified nucleic acid (N) having therapeutic efficacies and being rich in guanosine (G) is synthesized and the fact that the novel oligonucleotide-modified nucleic acid has excellent apoptotic activities on blood cancer cells and drug-resistant blood cancer cells is identified. Based on this, provided is a composition for preventing, ameliorating or treating blood cancer, containing the novel oligonucleotide-modified nucleic acid, and the novel oligonucleotide-modified nucleic acid or a pharmaceutically acceptable salt thereof as an active ingredient.
