The polyamide 12 (PA12) has very good processability, the lowest water absorption of all polyamides, with an exceptionally good dimensional stability and is the lightest of all polyamides, bringing great significance for applications in the fields of aviation and electronics, for example. To improve mechanical properties, like impact strength and brittleness, one solution is the addition of a polyolefin. This measure even reduces costs of processing. On the other hand, in order to avoid loss of its properties, the addition of fillers such as short carbon fibers (SCF) can aid in the balance between impact and tensile properties. The purpose of this work was the use of linear low-density polyethylene (LLDPE) as a second phase in the PA12 matrix and to improve the compatibility between the components, maleic anhydride grafted LLDPE (LLDPE-g-MA) was used as a compatibilizer agent for the blends. In search of the balance of mechanical and thermal properties, short carbon fibers were used. PA12/LLDPE/LLDPE-g-MA (70/25/5) blends with 5, 10, 15 and 30 wt% of SCF were prepared in the molten state using a homogenizer rotating at 3000 rpm (DRAIS, model MR-50H) followed by pressing of specimens (180 0C, 5 bar, 3 minutes). The composites were characterized by differential scanning calorimetry (DSC), tensile tests, onset oxidation temperature (OOT) and scanning electron microscopy (SEM). PA12/LLDPE is an immiscible blend and increasing the content of SCF in the composites increases the degree of crystallinity (Xc) of PA12 phase while the Xc of LLDPE phase remains unchanged. The onset oxidation temperature was about 430 oC for all compositions. The addition of LLDPE-g-MA controlled the size of the second phase and contributed to a good dispersion and distribution of the SCF in the composite. A substantial increase of E-modulus occurs with increasing the content of SCF, with an addition of 30 wt% of SCF the E-modulus had an increase of 60% compared to the blend.