The cathode microstructure of Li-air battery plays a significant role in Li2O2 storage and electron transfer. In this article, we report a tunable three-dimensional hierarchically macroporous carbon nanotube foam as the air cathode. Excellent electrochemical performance with a specific capacity over 10,000 mAh g(-1) is achieved, which can be attributed to a combination of sufficiently large tunnels for oxygen transport and an appropriate pore width for oxygen reduction reaction. Also, an inverse correlation between the size of major pores and cell capacities is observed experimentally and a simplified analytical model is proposed to explain this correlation. Based on a combined study of galvanostatic discharge, microscopy, porosimetry, and modeling, we find that the discharge product Li2O2 is mainly deposited in the major pores of CNT foams. (C) The Author(s) 2018. Published by ECS.