A method of the stress analysis is shown for linear, viscoelastic and aging materials, using an integro-differential operator. First we calculated the stress and the deflection of viscoelastic beam having an aging effect. Secondly the integro-differential operator was expanded into the operator induced from a tensor-valued function, and applied to the three-dimensional viscoelastic materials. Lastly composite rules were derived for viscoelastic composite materials having an aging effect.