This paper examined the effect of cellulose nanocrystal (CNC) content on water vapor permeability (WVP) and oxygen permeability (OP) of extrusion-blown poly(lactic acid) (PLA) films recorded at various temperatures and relative humidities. While both WVP and OP followed the Arrhenius equation, the WVP decreased with increasing temperature whereas an opposite trend was observed for OP, resulting in negative activation energy (E_p) for WVP but positive E_p for OP, regardless of CNC content. However, the E_p for both WVP and OP increased by adding CNC into PLA matrix. WVP was not affected by the relative humidity agreeing with Fick’s law. Nanocomposite films had lower WVP (∼40%) and OP (∼75%) than neat PLA films because of the tortuosity effect created by the presence of highly crystalline CNCs in the nanocomposites, which increased PLA’s degree of crystallinity. Indeed, the values of WVP and OP negatively correlated with increased crystallinity, irrespective of testing conditions.