Carbon foams have attracted much attention since they were first developed in 1960s. Most of the reported carbon foams are of thick cell walls, which do not make effective use of the carbon skeleton in application. Herein, we present a biomass-derived carbon foam with nano-thickness carbonaceous walls. Different from the conventional oven heating process, the carbon foam was prepared with the assistance of microwave irradiation within only a few minutes. The prepared carbon foam has reticular morphology. The meshes of the network are in the dimensions ranging from about 10-50 mu m and connected by thin carbonaceous walls with thickness of about 25 nm. The carbon foam has narrowly distributed slit mesopores around 2-4 nm. Active groups like hydroxyl, carbonyl, and epoxy groups are detected in its structure. It has potential to be applied as electrochemical material or catalyst support.