The performance of Li-air batteries with carbon nanotube and carbon nanofiber cathodes has been investigated using galvanostatic discharges at different current densities and scanning electron microscopy of the cathode electrode surfaces. It was found that the discharge capacity of the cathode electrode was mainly limited by the combination of oxygen diffusion and electrical resistance of the discharge product on the electrode surface. In addition, it was found that the specific capacity of Li-air batteries depends significantly on the discharge rates at the beginning and end of the discharge process: batteries that are being discharged at higher rates at the beginning and lower rates at the end show an approximately 10% higher specific capacity than batteries that are discharged with lower discharge rate at the beginning and higher discharge rate at the end. A novel mathematical model for the discharge of Li-air batteries with carbon nanotube and carbon nanofiber cathodes is also developed to describe the effects of the finite conductivity of the deposit layer in cathode. The model takes into consideration the fiber size distribution and the nonuniform deposition of the discharge product on fibers of different sizes and at different locations. The simulation results are in good agreement with the experimental findings and indicate that, although the effect of the deposit layer is to slightly decrease the power density of Li-air batteries, it can increase the total specific capacity of the battery. (C) The Author(s) 2014. Published by ECS. All rights reserved.