Mechanical properties of the AlSiMg/SiC and AlSiMgTiB/SiC composites produced by high pressure die casting (HPDC) are investigated. The mixture of ADC 11, master alloy AlMg, master alloy AlTiB and 99 % aluminium were used as metal matrix. A reinforcement particle was represented by high purity silicon carbide with an average particle size mesh 400. Aluminium matrix and SiC particle with the content: 5 %, 7.5 %, and 10 % wt were mixed by the semi-solid stir casting method. The stirring process was performed by 45o degree carbide impeller at a rotation of 600 rpm and temperature of 570 C for 15 minutes. The mixture of AlSiMg/SiC and AlSiMgTiB/SiC were shaped into the specimen by HPDC. Mechanical characterizations of composite specimens were done in hardness, tensile and impact tests. The density of the composites was also determined. The results have shown that adding SiC improves the hardness, tensile strength of the AlSiMg/SiC and AlSiMgTiB/SiC. The increase of % wt SiC decreases the impact resistance of the composites tested. The addition of TiB increases the hardness and ultimate tensile strength and ductility. A higher of % wt of SiC porosity of composite increases. This physical quantity was lower at the composite with TiB than without this type of ingredient. TiB caused grain refining of the matrix and enhances the mechanical properties of composites.