Aim: This in vitro study aimed at comparing the fracture resistance of microwave-cured high impact and conventional heat-cured acrylic resin mandibular denture bases, whether relined by a resilient liner or not. Materials and methods: Conventional heat-cured (C) and high impact microwave-cured (H) acrylic resins were used in the fabrication of 24 mandibular denture bases divided into 4 groups (n=6). Half of these bases were 4 mm in thickness to be used unrelined (CU and HU), the other half was prepared in reduced thickness (2 mm thick) to be relined (CR and HR) with addition silicon resilient liner (R). The denture bases were loaded in compression at the canine-premolar areas bilaterally until failure, using a universal testing machine. Data on fracture resistance (N), work of fracture (J) and deflection at fracture (mm), were analyzed by Kruskal-Wallis and Dunn's test (P≤0.05). Results: Among the unrelined (CU and HU) and the relined (CR and HR) bases, there were insignificant differences regarding the fracture resistance and the work of fracture median values. Fractures/cracks were repeatedly observed either unilaterally or bilaterally at the premolar regions of the bases. The CU bases exhibited the statistically lowest significant median deflection value at fracture compared to all other denture bases which were insignificantly different from each other. Conclusions: Relining of mandibular denture bases with resilient liners reduced their fracture resistance, work of fracture and increased their amount of deflection at fracture regardless to the type of the denture base material. Economically, conventional acrylic resin denture base material is still the material of choice for dentures that would require relining with resilient liners.