The significant influence of graphene oxide (GO) on the unidirectional foaming of poly(lactic acid) (PLA) using supercritical CO2 as blowing agent was investigated in this study for the first time. Highly oriented and elongated cell structures were obtained from the PLA/GO nanocomposites foams. The thermal, rheological, and CO2 absorption properties of the PLA/GO nanocomposites were studied to investigate the effect of GO on PLA unidirectional foaming. It was found that the incorporation of GO improved the storage modulus, loss modulus, and complex viscosity of the PLA/GO nanocomposites significantly. The addition of GO improved the CO2 absorption ability of the nanocomposites, which caused high expansion ratio and increased average cell size during foaming process. The high expansion force by enhanced CO2 absorption, high matrix viscosity of PLA/GO nanocomposites, and restriction of the mold in three directions together caused the formation of the highly elongated cell structure during foaming.