This paper proposes a wavelength assignment scheme for optical networks to reduce the number of wavelength converters in a condition that the number of wavelength is given. Wavelength conversion is needed for a purpose of reducing the number of required wavelengths in the network, since the number of wavelengths is limited. A parametric wavelength converter (PWC) is used as a wavelength converter because of its advantage of simultaneous multiple wavelengths conversion. The PWC uses a pump wavelength to define original and converted wavelengths. Position of the pump wavelength affects the converted wavelengths. As a result, the number of wavelengths depend on the position of the pump wavelength. The proposed scheme selects an output wavelength that the pump wavelength is near to the middle of the transmission wavelengths. Numerical results indicate up to 37% reduction in total converters required, comparing to that of the first-fit scheme.