Background and Objective: Designing a tooth preparation method for endocrown construction is effective in achieving clinical success. Considering that our knowledge about pulp chamber preparation is insufficient, this study was conducted with the aim of comparing the fracture strength of upper molar and premolar teeth treated with endocrown-reconstructed root based on two different methods of pulp chamber preparation. Methods: In this laboratory study, 15 upper molars and 15 premolars were divided into three groups. The first group (5 healthy teeth) was considered as the control group. The teeth of the second and third groups of root treatment were mounted in an acrylic JIG and their crowns were cut 2 mm above the CEJ. In the second group, the undercut of the pulp chamber wall was eliminated by a dental lathe. In the third group, the undercut was blocked by resin-modified glass ionomer (RMGI) cement. The teeth of the second and third groups were scanned by a laboratory scanner and restored and cemented with lithium disilicate glass ceramic endocrown (EMAXCAD, Ivoclar Vivadent). Then, all the teeth were subjected to a fracture strength test by a universal testing machine with a crosshead speed of 1 mm/min. Findings: The fracture strength of premolar teeth of group two (883. 160±, 186. 6) and group three (880. 380±, 262. 6) showed no significant difference. The fracture strength of molar teeth of group two (960. 120±, 444. 4) and group three (1290. 740±, 400. 7) was not significantly different. The mean fracture strength of healthy molars (2194. 400±, 375. 4) was significantly higher than restored molars (p<0. 05). In terms of fracture type (repairable/non-repairable), there was no significant difference between molar and premolar teeth in the studied groups. Conclusion: Based on the results of this study, two methods of eliminating the undercut or blocking out the pulp chamber with restorative materials can be used to make endocrowns in upper posterior teeth.