Titania has been frequently used as an electrorheological (ER) particle in ER fluids owing to its high dielectric constant. Additionally, titania possesses the ability to control the crystallinity range of particles. In this study, we investigated the effect of crystallinity control ability of titania on the performance of ER fluids, by adjusting the crystallinity of SiO2/TiO2 hollow nanoparticles (HNPs) through heat treatment at temperatures 200, 900, 1000, and 1100 °C. The morphologies of the pristine and heat-treated HNPs confirmed by transmission electron microscopy (TEM) images, and HNP treated at 1100 °C exhibited the largest grain size. The X-ray diffraction (XRD) patterns confirmed that as the annealing temperature increased, the crystallinity of titania increased, resulting in sharper, stronger characteristic peaks. Furthermore, it is revealed that variations in the crystalline structure and shell thickness induced changes in ER properties such as the loss tangent and charge accumulation. The loss tangent decreased with an increase in grain size considering the decrease in charge accumulation sites. To the best of our knowledge, this is the first study to report this novel insight, and it can be applied to subsequent studies for achieving high ER performance, and thus, realizing practical commercialization of ER fluids.