According to national standard method,the oil immersion test on the oilfield commonly used packer rubbers (nitrile rubber and hydrogenated nitrile rubber) was carried out,the stressstrain curves of oilimmersed rubbers were obtained by uniaxial tensile test.The results show that the break strength of immersed packer rubber is decreased by 20%,while the strainstress relationship is consistent,and the immerse packer rubbers are harden with the Increasing deformation.Correlation mechanism on the immersed packer rubber was studied using statistical mechanics of chain molecules.It is shown that the strain energy function of packer rubber is affected by the swelling of rubber after immersed and the strain caused by swelling after immersed.The analysis result of the change of strainstress relationship of rubber is consistent with the test result.This research lays the foundation of establishing constitutive relations of packer rubbers more reasonably.