Viscosity of crystal-bearing silicate melts is studied for isochemical systems in the range of linear and non-linear viscous flow. Effects on viscosity are investigated as a function of total volume fraction (0 - 35 vol%) and mean aspect ratio a:b:c (1:1:1.5 - 2.4:1:3.9) of suspended crystals. It is found that dispersed crystals increase the viscosity whereas shear thinning is attributed to alignment of crystals at moderate deformation rates.