This study presents some results on the mechanical behavior of polylactide (PLA) material, produced using the fused deposition modeling (FDM) additive manufacturing technique. We investigate the effect of infill density on the mechanical properties of PLA specimens. We used tensile specimens, prepared according to ISO 527-2 standard, and tested them by a universal testing machine with analysis by means of digital image correlation (DIC) method. The results in terms of UTS and nominal strain at break of PLA material are presented. They demonstrate a significant impact of infill density on material behavior of PLA specimens, as expected. Yet the effect is nonlinear that is indeed valuable to understand. As infill density increases, from 10% to 100%, the nominal strain at break decreases from about 2.1% to 1.2%, respectively. In other words, the material becomes more ductile by decreasing the infill density of PLA material, which is possible to justify with an effect of the microstructure created by the infill density. There is a transition of this observed behavior, from being more ductile to more brittle, by increasing the infill density of the PLA specimens.