High-strength aramid fibers have been used in the production of a variety of cables and ropes which have found several applications as structural members in bridges and other types of structures. For such applications, a sound knowledge of the time dependent material properties is highly important. An experimental investigation on the stress relaxation behavior of a high-strength aramid fiber was carried out, with the purpose of studying the viscoelastic properties of the material and the effect of a mechanical conditioning (application and removal of stress prior to testing) on these properties. The fiber used in this investigation is commercially known as Kevlar 49. The tests were conducted at five different stress levels, at constant temperature and moisture. The results showed that the fiber has a non-linear viscoelastic behavior and that the effect of the mechanical conditioning isn´t important. An equation for the prediction of the stress relaxation is presented.