The incorporation of micro-/nano-particles is one of the most efficient approaches to reinforce poly (lactic acid) (PLA). However, introducing the inorganic particles which can compromise the green nature of PLA. Herein, we proposed a green strategy to add biodegradable cellulose nanofibers (CNFs) into the PLA matrix for eliminating its low melt strength and slow crystallization rate. Well-dispersed hydrophobic-modified CNFs in the PLA matrix were obtained through the combination of acetylation surface modification and the melt-compounding technology. The PLA/CNF composites notably improved crystallization properties and melt elasticity, compared with the neat PLA. Additionally, the foaming behavior of PLA was dramatically enhanced by introducing 2 wt% modified CNFs. At an ultralow pressure of 1.5 MPa, the prepared PLA/CNF nanocomposite foams exhibited the highest expansion ratio approximately 20.4. These results demonstrated that CNFs were extraordinarily helpful in enhancing the foamability of PLA, which are expected to be applied as a sustainable packaging material.