The worldwide production of plastic products has increased significantly over the last decades and their disposal has become an important environmental concern. Recycling provides great opportunities for waste reduction and lower environmental impact. As a recycling alternative, there has been growing interest on addition of biomass residues to polymers to produce composite materials for technological applications. In this study, composites from recycled polyethylene waste and kapok fibers were prepared. Cold plasma treatment was used to improve fiber/matrix adhesion. The effects of plasma treatment on fiber morphology were evaluated using Field-emission gun scanning electronic microscopy (FEG-SEM) and on chemical structure of the fibers were evaluated by Fourier-transform infrared (FTIR) spectroscopy. Changes in water uptake (WU), mechanical properties and thermal decomposition kinetics behavior of the kapok fibers were studied. The results suggest an increase in water absorption after plasma treatment. Composites using untreated and treated fibers were manufactured and characterized. Dynamic mechanical thermal analyses (DMTA) of composites made using plasma treated fibers indicated an increase in storage moduli as well as an increase in glass transition regarding to the pure polymer. Thermogravimetric Analysis (TGA) indicated that the addition of plasma treated fiber produced changes in thermal degradation as compared to the pure polymer due to matrix/fiber interaction. Differential Scanning Calorimetry (DSC) analysis suggested that the addition of plasma treated fibers also produced changes in degree of crystallinity of the polymer. Field-emission gun scanning electron microscopy (FEG-SEM) images of fractured surfaces of the composites suggest that fiber/matrix adhesion was improved for composites made using plasma treated fibers. Thus, cold plasma is demonstrated as a viable alternative to treat cellulosic fibers and improve fiber/matrix interface of polymer-based composites.