Polylactic acid (PLA) foams were introduced as a good substitute for oil‐based foams. Much research has been done on the autoclave of PLA foam, but discontinuous processes are not attractive for industries. In order to reduce the distance between the PLA foaming process and industrialization, it is necessary to introduce continuous processes. In this work, two solutions to improve the foaming of PLA are presented: using the extrusion process and adding cellulose nanofibers. So, in this study, a twin screw extruder was used by blowing agent, CO₂, and in order to better dissolve the gas, a static mixer was used before the die. The molecular weight of the original PLA after extrusion has reduced in the range from 20%–28% due to the hydrolysis of the ester bond in the PLA. With the cellulose nanofiber loading, nucleation and cell density have enhanced. Scanning electronic microscope (SEM) microscopic images for the nanocomposites containing 1.5 wt% of cellulose nanofiber showed the highest cellular expansion and for the nanocomposites containing 2.0 wt% of cellulose nanofiber showed the most uniform cell distribution and it confirmed the density results and the calculated expansion ratio. Finally, by examining the dynamic‐mechanical thermal analysis, it can be stated that the composition of cellulose nanofibers improves the mechanical and dynamic properties of cellulose nanofiber‐reinforced PLA.