The kinetics of the electrochemical lithium insertion reaction in nano-sized rutile β-MnO2 has been investigated using ac impedance spectroscopy. The experimental kinetic data are obtained for a rutile compound synthesized by ball-milling the powder produced from the heat treatment of manganese nitrate salts. The results are discussed as a function of the Li content for 0< x <0.6 and the number of cycles in the 4.1–2V window. From a comparison with data obtained on the micro-sized oxide, an improved kinetics is found with D Li values for the apparent chemical diffusion coefficient of lithium much higher by one order of magnitude than in microsized oxide. Impedance behaviour of the ball-milled rutile β-MnO2 vs cycles demonstrates a new system takes place from the second cycle, characterized by a significant improvement of Li diffusion by a factor 5 and a cathode impedance which decreases by a factor 2, remaining thereafter unchanged during cycling.