Cationic peptide, poly-arginine, promotes transfection efficiency under a proper concentration. Using fluorescence microscopy, we observed the structure of the DNA carrier-complex in solution directly. Under low concentration of poly-arginine, small complexes were observed with fluorescence images like a blight light spot. This small complex (sizes are c.a. 1 approximately 2 microns order) had no transfection ability. As the concentration of poly-arginine became high, the complex grew into a big one whose size reached to a degree of 10 microns. Transfection was succeeded under that condition. The relationship between transfection efficiency and higher-order structure of the DNA carrier-complex was discussed.