Abstract: There is an absorption peak at 600 nm for toluidine blue(TB) in pH 4.9 acetate buffer solutions. The absorption value at 600 nm decreases with the concentration of ST, and the color become light. This is called hypochromic effect of the composite nanoparticle of (ST TB)_n. The nanoparticle system exhibits an absorption peak at 600 nm, three resonance scattering peaks at 400 nm, 470 nm and 580 nm. The resonance scattering at 400 nm is the characterization peak due to its quanta colored effect. The results of resonance scattering spectroscopy and the effect of some organic solvents such as alcohol, acetone show that the formation of the composite nanoparticle is the ultimate cause to produce the hypochromic effect.