Cow mattress is one of the industrial products related to agriculture. Cow mattress is used for cows safety when laying down and standing up, hard type of cow mattress is usually used for cows, and soft type for calves. This research aims to find out mechanical properties of cow mattress compound reinforced with carbon black as a filler varied with SIR-20 rubber: modulus 100%, tensile strength and elongation. The results showed that the highest value of modulus 100% was 2.81 MPa at the composition of carbon black is 40 bsk , and the lowest value of modulus 100% at without carbon black, the tensile strength of hard cow mattress reached the standard value (13.52 MPa) at the composition of black caron is 40 bsk and soft type reached at the composition without carbon black (10,17 MPa), elongation of hard type cow mattress reached standard value (473%) at the composition of black carbon is 30 bs and soft cow mattress at the composition of black carbon is40 bsk (424,33%).