The objective of this study is to investigate the possibility of small interfering RNA (siRNA) complexed with a cationized dextran of nonviral carrier to biologically modify the adipogenesis extent of bone marrow-derived mesenchymal stem cells (MSC). Spermine was chemically introduced to the hydroxyl groups of dextran to prepare the cationized dextran (spermine-dextran). The spermine-dextran could form a complex with siRNA, and the physicochemical properties were changed by the molecular weight of dextran, the molar percentage of spermine introduced to dextran, and the molar ratio of nitrogen molecule of spermine-dextran to the phosphorous ones of siRNA (N/P ratio). The gene expression level of luciferase or green fluorescence protein was significantly suppressed by transfection with the complex of spermine-dextran and siRNA. The gene expression level by the complex decreased with an increase in the extent of complex internalized. Biochemical experiments revealed that culture in an adipogenic differentiation medium allowed MSC to differentiate into adipogenic cells. However, upon culturing with siRNA of anti-transcription coactivator containing PDZ-binding motif (TAZ) for osteogenic differentiation complexed with the spermine-dextran, the adipogenesis of MSC was further promoted. It is concluded that the spemine-dextran was a promising nonviral carrier to suppress the expression level of differentiation gene, resulting in the modification of cell differentiation direction.