Steady shear viscosity of nylon 6 melts reinforced with xonotlite, microfibrous calcium silicate hydrate (6CaO . 6SiO(2) . H2O), is investigated. The highly filled nylon 6 melt tends to exhibit a yield value, resulting in remarkable viscosity increase particularly at low shear rates. Addition of the xonotlite significantly increases activation energy of viscous flow of the nylon 6 melt, leading the viscosity to be strongly temperature dependent. Comparisons with the melts filled with glass fibers and wollastonite are made. Flow-induced orientation becomes more important at low volume fractions. Shortening of the xonotlite during shear flow measurement can also be observed.