The effect of liquid viscosity on the internal fluidity of droplets during sliding on an inclined Si surface treated with fluoroalkylsilane (FAS) was investigated for water-glycerin mixture using particle image velocimetry (PIV). Increasing viscosity remarkably decreases the slipping motion on the solid-liquid interface and overall sliding acceleration. It affects the rolling motion, but its degree of influence is smaller than that on slipping. The viscosity had different contributions to slipping and rolling motions of the droplets.