A composite is a mixture of two or more different elements or phases. A phase that is continuous is called a matrix and the second phase is defined as a fixing phase, or a reinforcer (filler) that improves the mechanical properties of the matrix. This work investigates properties of pure polymer and polymer composites reinforced with natural filler-jute fiber. Polypropylene (PP), low density polyethylene (LDPE), polycaprolactone (PCL) and high impact polystyrene (HIPS) were used. The composite with 3 weight % of the filler were prepared by mixing in a Brabender mixer and pressing on a hydraulic press. Thermal (differential scanning calorimetry, DSC and thermogravimetric, TG analysis) and mechanical properties of pure polymers and obtained composites were determined and compared. The results obtained by DSC analysis showed that the addition of jute fiber promotes nucleation of PE-LD matrix and increase crystallization enthalpy of PE-LD, PP and PCL. The addition of jute fiber has no effect on the glass transiton temperature of HIPS. According to the values of TGA analysis, the addition of jute fiber increased the thermal stability of PE-LD, and decreased it in PP and PCL. HIPS addition of filler starts the decomposition process earlier, while the filler in the later stages of decomposition increased degradation temperatures. Mechanical properties of composites are different in relation to the polymer matrix. Modul and strength at break are increased with juta fiber addition.