Abstract: An improved plane strain test apparatus was first developed with some solutions for friction problem between load plate and specimen,control precision of ε2 and elimination of influence of confinement of both ends on volumetric strain measurement.A series of plane strain tests for sand were carried out together with triaxial compression tests and an elasto-plastic model was derived according to the test results.The elastic property,failure criteria,hardening parameter and hardening rule,and plastic potential function were all derived directly from the test results.It was shown again that the non-associated flow rule should be used for sand.Test results were simulated by the model using parameters derived from the tests,and the calculated stress-strain relation agreed well with the test results.