The mechanical properties of polymers, which depend on their molecular structure need to be known during the use of polymers. In this thesis the mechanical properties of high-density polyethylene (PE-HD), polypropylene (PP), polyvinyl chloride (PVC), polyvinyl acetate (PVAc), polystyrene (PS), poly (propylene-co-ethylene), (PPE), poly (ethylene-co-vinyl acetate), (EVA), poly (styrene-co-acrylonitrile), (SAN), poly (acrylonitrile-co-butadiene-co-styrene), (ABS) and ethylene propylene diene polymers (EPDM) were investigated and related to their structure. Tensile properties were tested with a uniaxial stress test and relaxation properties with a relaxation test. The tests were performed on a universal mechanical testing machine and the test samples were prepared on a hydraulic press. The mechanical properties depend on several factors of the polymer structure. They significantly depend on the shape of the macromolecule. Macromolecules can be linear, branched or crosslinked depending on the spatial arrangement and type of atoms. The PE-HD has a linear structure while EPDM has a crosslinked structure. The linear structure of PE-HD is dominated by crystalline phases and although it is in a viscoelastic state, it has a large stress at break and a large stress at the yield point . EPDM has a low degree of crosslinking due to which it has high elongation and low tensile stress. The mechanical properties also depend significantly on the type of substituent attached to the main chain in the repeating unit. For example, PS is a stiff material due to the impossibility of rotational freedom caused by a large benzene substituent in the repeated unit of PS while for example in PVC chloride substituent increases the glass temperature due to dipole - dipole bond between carbon and chlorine and thus change mechanical properties. Copolymerization also significantly changes the mechanical properties. For example, SAN was obtained by copolymerizing styrene and acrylonitrile. Acrylonitrile increases the stress in the material resulting in a large modulus of elasticity and a large stress at the breaking point.