Behavior of a package of a microwave oven under the clamping condition is evaluated by tests and the finite element analyses. PAM-CRASH software is used for the finite element analyses. Results of the analyses are compared with those of the tests and accuracy is shown to be favorable. Under the clamping condition of the original design, severe deformation occurs and an improved design of the outer case and upper EPS(Expandable Poly Styrene) is proposed to reduce it. Face beads of the outer case are introduced and shape of the upper EPS is modified to reduce the deformation resulting from the clamping load. The improved design model is analyzed and its deformation is shown to be satisfactory. A prototype is produced according to the improved design and tests are performed. Results of the clamping test of the prototype show that the plastic deformation is removed totally.