Polymer blends of poly(lactic acid) (PLA) and polybutylene succinate (PBS) containing activated carbon (AC) were foamed through an extrusion process using azodicarbonamide (ADC) as a blowing agent. The influences of PLA:PBS ratios and ADC and AC loadings on the bubble formation, cellular structure, foam density, void fraction, cell density, melt viscosity and crystallinity were investigated. PBS acted as nucleation sites for the cellular generation at low concentrations. The thicker cell walls occurred with smaller cell sizes and higher cell density at high PBS contents. The crystallinity (Xc) of each polymer in composite foams was lower than that of un-foamed polymers and declined with the reduction of polymer proportions. The production of gas bubbles increased with the increase of ADC contents. However, the cell size tended to decrease at high ADC dosage due to low melt strength and high gas pressure. The Xc of PLA in composite foams decreased when ADC contents increased and then increased at high ADC dosage. The Xc of PBS in composite foams showed the opposite results to Xc of PLA. The reduction of Xc of PLA assisted the increment of gas bubbles coalescence and cell expansion. At low AC contents, the distribution of fillers was good and the AC acted as a nucleating agent for cellular formation and crystallization of PLA. However, AC exhibited a higher tendency to agglomerate at high concentrations. The increase of AC contents mostly affected the Xc of PLA in composite foams while the Xc of PBS slightly changed at varied AC concentrations. The balance of melt viscosity, nucleating effect, filler distribution, gas pressure and Xc of polymers was required during the foaming process to obtain optimum cellular formation. The composite foam containing PLA:PBS ratio of 80:20 wt%, 5 phr of ADC and 3 phr of AC possessed closed microcellular structure with high uniformity of cell size and distribution, including the maximum reduction of foam density of about 50%.