Introduction: Alveolar bone resorption, under free end removable partial dentures bases, has been considered one of most common problems for both of the dentist and the patient, though the multiple studies and techniques have been achieved to reduce this problem. Aim of The Study: this research aimed to measure the alveolar bone resorption under cast metal removable partial dentures with flexible acrylic resin bases, according to class I Kennedy depending on two different methods of final impression. Materials and Methods: (15) mandibular removable partial denturs of calss I kennedy have been made for (15) patients using McLean physiologic impression and Selective pressure single impression, then a digital panoramic radiography was made for each patient while was wearing the denture, after artificial tooth had been restored by dental Amalgam to be a reference point for measurement. Aafter (6) months the panoramic image has been taken again and the measurement has been done by the Adobe photoshop program. Results: Within the limits of the study showed; The occurrence of alveolar bone resorption with time in dissimilar rate between patients, whatever the method of functional impression was used, but with no statistically significant differences, and showed the repcclication of resorption in the most distal area from the distal abutment in selective pressure single impression group comparing to Mclean physiologic impression group with statistically significant differences. Conclusions: There is no effect of the method of the functional impression in alveolar bone resorption under cast metal removable partial dentures with flexible acrylic resin base, with preferable to use the Mclean physiologic impression for this type of dentures.