Study of drag force on dendritic particles in highly viscous fluids is essential for understanding the dynamic sinking/floating behavior of particles in metallurgical systems. In this study, experiments were carried out to investigate the drag coefficient of simplified three-dimensional dendritic particles in silicone oil with a range of viscosities (20, 100 and 500 cSt). The experimental system was designed to represent the basic oxygen steelmaking (BOS) slag system based on a dynamic similarity approach. The drag coefficient of each dendritic particle was calculated from the measured terminal velocity based on a force balance and the known drag coefficient of its volume equivalent sphere. The drag coefficients were determined over a range of Reynolds numbers (~ 0.035 to 84) differing by two orders of magnitude accounting for various particle shape factors that included aspect ratio, sphericity, and particle orientation. Correlations between the drag coefficient and the shape factors were proposed for each viscosity.