Antisense therapy using oligonucleotides has attracted many attentions because of its high efficiency of manipulating the expression of disease-causing genes. To endow the therapeutic molecules with an ability to target specific cells, oligonucleotides have been conjugated to targeting molecules such as ligands and antibodies. In particular, the antibody has been a focus of research because of its pharmacokinetic and physicochemical features. However, these conjugate molecules still have a limitation of low efficiency to locate in the cytoplasm or nucleus where the oligonucleotide works. In this study, I investigated the capability of cytotransmab, which can internalize via endocytosis and locate in the cytoplasm by escaping the endosome, to deliver oligonucleotides. The thiol group of unpaired Cys residues were used for conjugation with oligonucleotides modified with a linker having the maleimide group. Several positions of IgG were evaluated for the reduction and re-oxidation steps to generate unmodified thiol groups in the introduced Cys residues, and position N425 in the heavy chain was chosen for substitution with Cys. Single- or double-stranded oligonucleotides with different length were conjugated to cytotransmab, and binding affinity and cytoplasmic localization of these conjugates were analyzed using flow cytometry and split GFP complimentary assay, respectively. The reduction, re-oxidation and oligonucleotides were not affect the affinity of inCT99 (N424C)-GFP11 against integrin αvβ5. In addition, it was shown that conjugates can translocate into cytosol, and the length of oligonucleotide is a factor which can affect the escaping from endosome of cytotransmab.