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Nano sized LiNi0.5-2xRuxMn1.5O4 (x=0, 0.01, 0.03 and 0.05) spinels have been successfully synthesized by polymer assisted method. SEM observation shows that particles' sizes of LiNi0.5-2xRuxMn1.5O4 are around 300 nm. XRD and FTIR measurements prove that all Ru doped samples have phase pure spinel structure with Fd3m space group, while pristine LiNi0.5Mn1.5O4 contains impurity phase (LixNi1-xO). The nano sized LiNi0.5Mn1.5O4 can release 117 mAh. g-1 at 10 C discharge rate; while, the nano sized LiNi0.4Ru0.05Mn1.5O4 can deliver 135 mAh. g-1 at 10 C discharge rate. In 500 cycles 10 C charge/discharge test, the nano sized LiNi0.4Ru0.05Mn1.5O4 can initially deliver near 121 mAh. g-1 and still keep 100 mAh. g-1 at the 500th cycle exhibiting much better cyclic performance than the pristine LiNi0.5Mn1.5O4. It also exhibits excellent cyclic performances when charge/discharged at 10C for 1000 cycles. The great improvements on high rate performances of Ru doped cathodes can be attributed to their higher electronic conductivity and lithium diffusion coefficient, which are achieved through both Ru doping and reduction of particle size.