Aim: This study aimed to compare the effect of different remaining dentin thicknesses, between floor of pulp chamber and furcation area, alone or after addition of MTA layers on fracture resistance of mandibular molars. Materials and methods: Access cavities were performed on fifty human extracted mandibular molars. Teeth were randomly assigned into 3 main groups. Group 1: remaining dentin thickness 2 mm (n=20). Group 2: remaining dentin thickness 1 mm (n=20). Group 3: average dentin thickness 3 mm between the floor of pulp chamber and bifurcation area (n=10). Group 1 and 2 were sub-divided into two equal subgroups (n=10). For group 1: Subgroup (1a): no MTA was added and subgroup (1b): 1 mm of MTA was added. For group 2: Subgroup (2a): no MTA was added and subgroup (2b): 2 mm of MTA was added. Then, a compressive force was applied toward the floor of the cavity causing vertical fracture by using Instron testing machine to determine the remaining dentin thickness behaviour in all groups and their subgroups. Results: There was no significant difference between (G: 1a, 2a and 3) with p-values 0.54, 0.67 and 0.99 respectively. As well as no significant difference between Group 2b and Group 1b with p-value 0.12. Conversely, Group 2b and other groups (Group 3, 1a and 2a), there were statistically significant differences with P-values= 0.05, 0.04 and 0.006, respectively. Conclusion: Addition of a layer of 2 mm thickness of MTA to the floor of pulp chamber significantly increases the fracture resistance at bifurcation area.