Abstract Polymer composites with cellulose microfibrils are materials with unique properties compared to existing materials: high strength, formability, and geometrical complexity at very small scale, low density, and abrasivity, biomedical compatibility, and possibility of recycling. Polymer composites from polypropylene and two types of cellulose microfibrils were prepared and characterized. To enhance the compatibility of cellulose microfibrils with polypropylene matrix, a silane‐coupling agent and a modified polypropylene were used. Depending on the applied method, composite materials with enhanced mechanical properties were obtained. Morphostructural properties of cellulose microfibrils and polymer composites with treated and untreated microfibrils were investigated using scanning electronic microscopy (SEM), FTIR spectroscopy, and differential scanning calorimetry (DSC). POLYM. ENG. SCI., 47:1228–1234, 2007. © 2007 Society of Plastics Engineers