The effective non-linear parameter of a highly non-linear optical fiber was measured using the four-wave mixing effect. A value of γ = 10.6 W < sup > -1 < /sup > /km was obtained for a small wavelength separation between pump and signal waves, whereas for a large wavelength separation, a value of = 8γ/9 = 9.4 W < sup > -1 < /sup > /km was found. This value corresponds to an average of the Kerr non-linearity over the Poincaré sphere, since the polarization of both the pump and signal waves become uncorrelated in the last case. A theoretical model is proposed to describe the variation of the effective non-linear parameter with the wavelength separation between pump and signal waves, which is in good agreement with the experimental data.