Twelve patients representing mandibular Kennedy class I dentition, were categorized into two groups, according to denture base material of the dentures introduced in the treatment plan: Group 1: received a removable partial denture made of conventional heat cured acrylic resin denture base material with wrought wire clasps. Group 2: received a removable partial denture made of flexible thermoplastic nylon denture base material with flexible clasps (Bre.flex) The abutment teeth were evaluated radiographically using digital radiography (Digora system) to assess the effect on marginal alveolar bone height of teeth abutments. The evaluation was established at the time of delivery (Baseline), one month, three months and six months later. Results of the study showed slight statistically significant difference in favor of flexible thermoplastic nylon denture base material group. Change percentage in alveolar bone height showed higher percentage of bone height decrease in the group received conventional heat cured acrylic resin denture base, which means that this type of material causes a greater harmful effect to the supporting structure of the abutments studied. Based on results presented in this study, it can be concluded that: Thermoplastic nylon, in provisional prosthesis, can be used as a successful denture base material showing superior esthetical and biomechanical characteristics than heat cured acrylic resin.