Statement of problem: Polymethyl-methacrylate is still a widely used material for complete and partial denture fabrication. Candida biofilm adherence to PMMA denture base has been studied, and ways to control its adherence by using different ways of cleaning, natural and chemical herbal cleanser-based methods have been introduced as an alternative to chemical methods. Using any type of cleanser may affect the flexure strength of denture bases. Purpose: This study aimed to compare the effect of herbal extract cleanser and chemical cleanser on the flexure strength of three different denture base materials: heat polymerized acrylic resin, thermoplastic polyamide (Nylon), and thermoplastic monomer-free microcrystalline polymer (Karadent). Materials and methods: A total of 90 samples were divided into three equal groups (Heat cured processed acrylic resin, Karadent, Nylon) Samples were immersed in the cleansing solution and distilled water as a control group for 15 days. Results: Flexure strength was measured using a universal testing machine, and the results were collected, tabulated, and statistically analyzed using ANOVA. There was a significant difference between thyme-immersed specimens and specimens immersed in sodium hypochlorite (NaoCl). The Karadent specimens showed the least flexure strength measures among the three groups. Conclusion: immersing in different cleanser affected the flexure strength of denture base materials, using thyme as a cleanser has less effect on denture base flexure strength in comparison to NaoCl.