The effect of ethylene-vinyl acetate (EVA) modified with mercaptoacetic acid on mechanical, thermal, and morphological properties of blends of natural rubber (NR) and EVA copolymers has been investigated. The introduction of EVASH promotes a crosslinking of natural rubber phase as indicated by extraction experiments and microscopy analysis. This crosslinking may be attributed to bonding between sulfhydryl groups along the EVASH backbone and double bonds in rubber phase and may be responsible for the hardness improvement of most of the studied blends. Better results on hardness and ultimate tensile strength with EVASH addition were achieved for NR-EVA (60:40) ratio, probably due to cocontinuous morphology of this composition. The influence of EVASH on crystallinity degree of NR-EVA blends was also studied by differential scanning calorimetry. The morphology of the blends was studied through scanning electronic microscopy.