Bolts and bolted connections are the most common compounds that are used in almost all industries, especially in the car industry. Because they are so commonly used, they represent a certain challenge to find a way to stop their loosening and to detect what is their pretension loss due to various influences. In recent times, the car industry is turning to manufacture hybrid and electrical vehicles more and more. Since they are turning to that segment of production, various batteries, which first have to be tested before “the real time use” are developing. Bolts connect many parts in all batteries, and because of that, it is necessary to investigate how much those bolts are able to keep 100% of their initial pretension force, under various conditions. Also, in the world of the bolts and the bolted connections it is very familiar when certain pretension force is applied, it starts to drop immediately. Because of the viscoelastic behavior of the underlaying material, that effect of pretension loss exists. Considering all of that, in this thesis using the theoretical background and finite element method it is going to be investigated what is the pretension loss due to the viscoelastic behavior of the material with which the bolt and the nut are connected. Because of that, it is necessary to develop a method, which will be able to examine how relaxing of the bolted connection is influencing the pretension and which will be usable when real tests are done.