Aluminium fly ash metal matrix composites (MMCs) find important applications in aerospace and automobiles where specific stiffness is important. Low cost fly ash and silicon carbide reinforcement are widely used in aluminium metal and matrix composite due to its low density, high young modulus and strength apart from good mechanical and chemical compatibility & thermal stability. However the MMCs often suffer from low ductility, toughness and fatigue crack growth resistance relative to the matrix alloy. Linear elastic fracture mechanics (LEFM) has been used to characterize the plane strain fracture toughness using various specimen geometries and notches but very few studies using EPFM are reported in literature. In the present paper the influences of weight fraction of particulate reinforcement on tensile, fracture toughness have been evaluated. The tensile strength of aluminium fly ash composites increases with the addition of fly ash reinforcement. However the fracture toughness (KIC) of the aluminium fly ash composite decreases that of base alloy. The fracture toughness KQ of AA6061 ALFA composites varied between 13-14 MPa/m as compared to 18 MPa/m for the re-melted base alloy. Similarly the Elastic plastic fracture toughness JQ for the base alloy AA6061 lies is the range of 20-23 kJ/m 2 and that of composites in the range 6-16 kJ/m 2 . The fracture behavior and micro-mechanism of failure in base alloy and composites have been observed under SEM and optical microscopy.