In this work, the effect of the incorporation of leather wastes from chrome tanning on the mechanical and rheometric properties of natural rubber-based composite was evaluated. Chrome content in leather wastes diminished as a result of a chemical treatment with a sodium bicarbonate solution. The treatment also improved the tensile properties of composites with 0, 20 and 40 parts per hundred rubber of leather wastes with particle size between 590 and 840 µm. In a second stage of the experimental work, 0, 20, 40, 60, 80 and 100 parts per hundred rubber of chemically treated leather wastes with particle size between 297 and 590 µm were incorporated to the same natural rubber compound studied in the first part of this study. All the composites were prepared in a torque rheometer, seeking to determine their rheological behaviour during mixing. Rheometric properties of all the composites during curing were monitored in a moving die rheometer. Tensile properties, tear strength, hardness, compressive modulus, compression-set, abrasive wear resistance and De Mattia flexing fatigue resistance for composites obtained in the second experimental set of this work were evaluated. It was found that leather wastes improved the hardness, stiffness, tear strength and abrasive wear resistance on the composites. However, elongation at break, flexural fatigue and tensile strength diminished when a large quantity of wastes was incorporated.