PLA is one of the most widely used materials in additive technology, especially in the Fused Filament Fabrication method. However, its use is limited by its relatively low-temperature resistance. The glass transition temperature of PLA is around 60 °C. Some studies already performed on the annealing of 3D printed plastics have shown their increased heat deflection temperature. However, the interpretation of some of these studies' results does not correspond to our practical experience. For example, one of those works reports that annealed PLA samples soften at much higher temperatures than the original nonannealed ones. This paper intends to better understand the behaviour of annealed printed PLA samples at higher temperatures and extends previously performed studies to determine glass transition temperatures using TGA. The samples are tested in two devices to determine the temperature resistance. The first is based on the standard for determining the heat deflection temperature according to ISO 75. In the second device, the sample is subjected to tensile stress at a specific temperature under the simultaneous action of stress induced from the screw connection. The results show that the annealed samples' glass transition temperature does not differ significantly compared to the nonannealed samples. Furthermore, tests have shown that annealed samples deform under relatively small load at higher temperatures than nonannealed samples, but this deformation depends on its magnitude. If the load is high enough, the annealed specimens deform at the same temperature as the nonannealed specimens.