Polyoxymethylene composites with octakis (dimethylsiloxy, ethyl epoxycyclohexy) octasilsesquioxanes and octakis(dimethylsiloxy, vinyl) octasilsesquioxanes contents (0.5 and 1 wt.%) were studied. The effect of silsesquioxanes (POSS) on crystallization and melting behaviour of polyoxymethylene (POM) was investigated by polarized light microscope (PLM), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), with the aim to determine the structural properties like crystallization temperature, degree of crystallinity and lamellar thickness distributions. It was found that POSS as nucleating agent affects the crystallization process of POM, leading to a homogeneous structure, improving the thermo-mechanical properties of polyoxymethylene composites. Based on SEM observations it was found that the POSS particles in a concentration of 0.5 wt.% are uniformly dispersed in polyoxymethylene matrix. The storage modulus and relaxations temperatures of POM with POM/0.5 wt.% ehPOSS and vinPOSS are slightly increased.