Polylactide acid (PLA) polymers have attracted enormous attention as a replacement for conventional synthetic packaging materials in the last decades. Blends of PLA with several synthetic and biopolymers have been prepared in an effort to enhance its mechanical and processing properties. Polypropylene (PP) is not biodegradable, but possesses superior mechanical strength, resistance to chemical agents as well as low cost. Several studies were carried out on blends of PLA and PP to analyze their morphology and mechanical behavior. However, since PP and PLA form immiscible polymer blends, they are suitable candidates for the development of in-situ microfiber-reinforced composites (MRC) which can be obtained through the processing of immiscible polymers having different melting temperatures. In this study, we report on the incorporation of PP fibres into PLA matrix to obtain MRC prepared through melt mixing, slit die extrusion and cold-stretching. The slit die extrusion was performed at the processing temperature of PLA matrix, below the melting temperature of PP, in order to allow the fiber production by cold-stretching of the polypropylenic phase. Due to their immiscibility feature, the interfacial interaction between PP microfiber and PLA matrix is poor. In order to reduce the interfacial tension of the blend, the use of a suitable compatibilizer is often required. To this aim, in the present work PP-grafted with maleic anhydride was used as an interfacial compatibilizer. Results show the effect of the compatibilizer to stabilize the morphology developed during compounding and to enhance adhesion between the phases thus improving the mechanical and barrier properties of the obtained PP/PLA microfiber reinforced composite.