Instantaneous response time is an important parameter of electrorheological (ER) fluids.Based on the expetiment and theoretical analyses, a model of response time of ER fluid re garding the geometrical factor of dispersed phase particles is established. According to the model, for the same particle volume of the dispersed phase particles the response time of the ER fluid which consists of prolate ellipsoid particles is shorter than that of the ER fluid which consists of sphere-like particles, while the response time of the ER fluid which consists of oblate ellipsoid or laminar-like particles is longer than that of the ER fluid which consists of sphere-like particles, and the response time of the ER fluid which contains rod-like particles is shorter or longer than that of the ER fluid which contains sphere-like particles depending on the value of εp/εf. The relationship between the response time and the size of particles isthat (1) with the increase of the particle volume, the response time of ER fluids containing different shape particles decrease; (2 ) under the same volume, with the increase of the prolateradius ratio, the response time of ER fluid containing prolate ellipsoid particles decreases,while the response time of ER fluid containing oblate ellipsoid or larninar-like particles in creases, and the response time of ER fluid containing rod-like particles increases or decreasesin accordance with the value of εp/εf. In order to get ER fluid with short response time, be sides sphere-like particles other shapes such as prolate ellipsoid and rod-like particles with a given size can be choosed at a low Reynold number.