The constitutive equations for epoxy compounds used for Integrated Circuits (ICs) were examined in a small viscoplastic strain region of approximately 0.01 in the temperature range from room temperature to the glass transition temperature. The specimens that were moulded and cured were subjected to tensile testing to examine their constitutive equations. Various characteristics obtained in the tensile test were reasonably represented using a simple model in which a three-element viscoelastic model, which changes depending on viscoplastic strain, is connected in series with conventionally used viscoplastic elements. The loading and unloading characteristics of specimens were also evaluated with satisfactory accuracy using the model.