Alkaline peroxide is effective and the most widely used denture cleanser. However, some studies encountered the disadvantage of the oxidative process results that affected the chemical and physical properties of denture base. Thus, castor oil has been studied as herbal denture cleanser with bactericidal and fungicidal effect without changes in water sorption and color stability of denture base. High water sorption and low color stability are problems that mostly encountered in themoplastic nylon material. Therefore, polyetheretherketone (PEEK) has been introduced as denture base material. The purpose of this study was to observe the effect of immersion of thermoplastic nylon and polyetheretherketone denture base in alkaline peroxide and castor oil denture cleanser on water sorption and color stability. Thirty samples with a diamater of 15 mm and thickness of 1 mm were used in this study. Samples were divided into six groups (n=5), such as thermoplastic nylon and polyetheretherketone that were each immersed in distilled water, alkaline peroxide and 10% castor oil denture cleanser. Water sorption test was weighed using digital analytical balance and calculated into percentage using Kazanji and Watkinson method. Color stability test was performed using colorimeter and calculated based on the formula (ΔE). Unpaired T Test showed significant difference on water sorption and color stability between thermoplastic nylon and polyetheretherketone denture base after immersion in each alkaline peroxide and 10% castor oil denture cleanser with p value =0.0001 (p<0.05). Besides, One way Anova test also showed significant effect on water sorption and color stability in each nylon thermoplastic and polyethertetherketone after immersion in distilled water, alkaline peroksida and 10% castor oil with p value =0.0001 (p<0.05). Based on the results of this study, it could be concluded that polyetheretherketone denture base could be suggested for dentist clinical used due to its good chemical and physical properties. Besides, castor oil 10% denture cleanser could also be used as denture cleanser due to its low water sorption and good color stability compared to alkaline peroxide on thermoplastic nylon and polyethertherketone denture base.