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At present, the combination of metal matrices and ceramic reinforcement as composite is popular for creating materials with good mechanical properties. Aluminum is used as a matrix because of its lightweight, ductile, and low melting point. In this paper we discuss the effect of adding silicon nitride reinforcement to AC4B aluminum. Five sample variations were made based on fraction volume percent (1, 3, 5, 7, 10) which were fabricated through stir casting method. In the fabrication process the magnesium element was added to increase the wettability, TiB grain refiner, and modification with strontium element. The samples made got destructive, tensile, impact, and hardness testing to determine the mechanical properties of the composite. Observation of phases, elements, compounds and microstructure was done to see the distribution of reinforcing particles in the aluminum matrix and the estimated phases formed. The results showed that the optimum tensile strength of variable occurred at the addition of 1% volume of silicon nitride with a strength value of 104.94, and the optimum hardness value was at the addition of 3% Si3N4, which was 44.8 HRB. The phases formed in the composite were Mg2Si, Al2Cu, β-Al5FeSi, and π - Al9FeMg3Si5 phases.