Understanding　the　degradation　of　material　properties　and　stress-strain　behavior　of　rubberlike　materials　that　have　been　exposed　to　elevated　temperature　is　essential　for　rubber　components　design　and　life　time　prediction.　The　complexity　of　the　relationship　between　hyperelastic　materials,　crosslinking　density　(CLD),　and　chemical　composition　presents　a　difficult　problem　for　the　accurate　prediction　of　mechanical　properties　under　thermal　aging.　In　this　paper,　a　new　and　relatively　simple　mathematical　formulation　is　presented　to　expresses　the　change　in　material　properties　of　hyperelastic　materials　under　thermal　aging.　The　proposed　formulation　has　been　applied　to　a　natural　rubber　(NR).　Testing　was　performed　on　more　than　130　specimens　that　were　thermally　aged　then　subjected　uniaxial　tension　and　hardness　tests.　The　aging　temperatures　ranged　from　76.7　degrees　C　to　115.5　degrees　C,　and　the　aging　times　ranged　from　0　to　600　h.　Based　on　the　recorded　experimental　data,　the　NR　mechanical　properties　under　thermal　aging　showed　a　similar　behavior　to　the　rate　of　change　of　the　CLD　with　aging　time　and　temperature.　Three　mechanical　properties　have　been　chosen　to　be　studied　in　this　paper:　the　ultimate　tensile　strength,　the　fracture　stretch　value,　and　the　secant　modulus　at　11.0%　strain.　The　proposed　mathematical　formulation　is　a　phenomenological　equation　that　relates　the　material　properties　with　the　change　in　CLD　based　on　a　form　of　Arrhenius　equation.　The　proposed　equation　showed　promising　results　compared　to　the　experimental　data　with　an　acceptable　error　margin　of　less　than　10%　in　most　of　the　cases　studied.