Purpose: This study investigated the peel strength of two different soft liners to a polymethylmethacrylate (PMMA) denture base resin before and after thermocycling. Materials and Methods: The silicone-based soft liner materials tested were Molloplast B and Permaflex; the denture base material was a heat-cured acrylic resin, Meliodent. A total of 40 specimens was prepared using rectangular molds with dimensions of 100 x 10 x 2 mm for PMMA and 150 x 10 x 2 mm for soft liners, as described in ASTM-D903-93. For each of the liner materials, 10 specimens were packed against a cured PMMA denture base surface as recommended by the manufacturers. The other 10 specimens were packed against PMMA denture base dough and processed together. In each group, 5 of the specimens were tested directly, while the other 5 were thermocycled in a water bath (5°C to 55°C; 3000 cycles) before testing. Peel testing was performed using an Instron testing machine. Results: The results revealed that peel strength values of the Permaflex specimens prepared according to the manufacturer's recommendations were significantly higher than those of Molloplast B (p 0.05). However, when packing was done against uncured PMMA dough, the difference between the specimens of two liners was not significant. Thermocycling led to significant decreases in the peel strength of both Permaflex liner specimens packed against cured/uncured PMMA resin surfaces (p 0.05), whereas this process did not affect the strength of Molloplast B specimens. Conclusion: Results indicated that the material Molloplast B was superior to the material Permaflex in terms of peel strength when the specimens were simultaneously polymerized with PMMA and thermocycled. Keywords: peel strength, PMMA, silicone soft liners, thermocycling