This paper introduces a method for estimation of the radiative properties of a fibrous medium, taking account of fiber orientation. A formulation for deriving the radiative properties the extinction and scattering coefficients and scattering phase function of the medium is described for when the complex refractive index of the fiber material is known. For the case when the complex refractive index is unknown, an estimation method for the scattering phase function of the medium is described. Here, the radiation scattered by a single fiber removed from the fibrous medium is measured and the phase function is derived from the measurement results using the above formulation. The phase function for a slab with fiberglass randomly oriented in a plane was estimated by the present method. The radiation transfer in the slab, calculated using the phase function of the slab, is in good agreement with the transfer measured by experiment.