The rate capacity especially the high rate discharge performance is another important aspect for the application of Mn-based spinel cathodes for EV/HEV power sources besides the cycling performance that is now intensively investigated. In this paper, spinel materials differing in chemical composition and thermal processing history were investigated by discharging at constant current rates from C/10 to 4C at ambient temperature. It was found that the high-rate discharge capability of Mn-based spinels is very excellent if prepared at temperatures below 850°C, no matter cation doping or not. In contrast, spinels synthesized over 950°C showed much poorer high rate performance, and some kinds of impurities were proposed to be responsible for the deteriorated behavior. Annealing at lower temperatures was found to be useful for the significant improvement of the high rate discharge capability of Mn spinels.