The research and development of new materials that are not only functional, but also ecologically acceptable, is a key aspect in many branches of industry. Such materials include elastomeric composites reinforced with alternative fillers such as cellulose. Cellulose is a renewable and biodegradable alternative to traditional fillers used in elastomeric composites. The main disadvantage of this biopolymer is its poor compatibility with the hydrophobic matrix and low mechanical strength. The free hydroxyl groups on the cellulose surface allow for a wide range of surface modifications. In this work, we focused on the chemical modification of cellulose using two different silanes due to their ability to react with the free hydroxyl groups on the surface of cellulose. This work deals with the characterisation of thermal stability of surface modified cellulose used as filler in polymer composites. Cellulose modified in this way was used in the amount of 45 phr as a filler in the preparation of elastomeric composites with natural rubber (NR) matrix. The NR composite filled with surface modified cellulose was characterized by TG/DSC, IR spectroscopy, XRD and scanning electron microscopy.