To develop efficient gene carrier, chitosan oligosaccharides (COSs) were chemically modified with hydrophobic moiety of deoxycholic acid (DOCA). The physicochemical properties of the hydrophobized CSs (COSDs) were investigated by using dynamic light scattering and fluorescence spectroscopy. COSDs formed nano-sized particles in aqueous milieu by hydrophobic interaction of conjugated DOCA. The MW of COS and degree of substitution (DS) were played critical roles for the physicochemical characteristics of COSD nanoparticles. Than COSDs were complexed with DNA, and COSD/DNA complexes were characterized. Compared to the unmodified COSs, the COSDs efficiently condensed DNA and protected the condensed DNA from DNase I attack. Furthermore, the COSDs/DNA complexes showed about 5 times enhanced transfection effciencies than unmodified COSs. Therefore, hydrophobized chitosan nanoparticles has much potential for effcient gene carrier.