Despite its many advantages, poly(lactic acid)'s (PLA's) vast commercialization as a packaging material has been hindered by its numerous drawbacks such as low melt elasticity, slow crystallization, and low gas barrier properties. In this pioneering research, we introduce the concept of in situ microfibrillation process as an efficient, cost-effective, and environmentally friendly technique for the enhancement of PLA films' properties. In situ microfibrillar composites (MFCs) with PLA matrix and different (petroleum and biobased) polyamide (PA) microfibrils were produced via a fast and simple melt extrusion and hot stretching process. Morphological observations demonstrated the successful transformation of the dispersed PA phase into very long and flexible microfibrillar shapes with diameters of nearly 200 nm. Shear rheological investigations proved that the MFCs had dramatically improved melt elasticities compared with the pure PLA. The crystallization kinetics of the PLA was also significantly improved after the microfibrillation process which was attributed to the heterogeneous crystal nucleation effects of the microfibrils. Gas permeability measurements confirmed substantial reductions in the oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) of the PLA film after the microfibrillation process followed by isothermal annealing. The MFCs also showed significantly improved tensile properties in comparison with the PLA. (C) 2016 Elsevier B.V. All rights reserved.