After organic gas treatment for 25 hrs in 1,2-Dichloroethane vapor, the third order nonlinear susceptibility of PMMA/Tertiary-butyl Vanadylphthalocyanine [(t-Bu)nVOPc] composite film was observed to increase markedly because the phase morphology of PMMA/(t-Bu)nVOPc composite film changed from Phase I to II. A waveguide was prepared with a triangular prism and PMMA/ (t-Bu)nVOPc composite film treated for 25 hrs in organic gas. When irradiating the waveguide with input laser power, the output power was recognized as having optical bistability. Moreover, the input laser power dependences of optical bistability were measured using the waveguide. Their optical bistability had a higher stability and sensitivity than that of the optical bistability reported previously and excellent reproducibility, which meant it was not necessary to write the error bars. However, in the input laser power dependences of optical bistability, the on-off switching position shifts according to the input laser power intensity. This indicates that by changing the input laser power intensity, the thickness of the PMMA/(t-Bu)nVOPc composite film increases due to the heat occurring when laser light is irradiated onto the composite film, leading to a marked enhancement in the third order nonlinear optical susceptibility (χ(3)), and a subsequent increase in the nonlinear optical refractive index of the PMMA/(t-Bu)nVOPc composite film, linked to an enhanced χ(3) value. Therefore, the increase in the χ(3) value is closely related to the changing switch on-off position, according to the input laser power intensity.