Ribbond fibers are supposed to be a reinforcing material in restoration of compromised teeth. This study aims to compare MOD restorations with and without Ribbond Fiber in terms of fracture strength under axial loading; to identify the minimum depth of MOD cavities to use Ribbond Fiber (to improve the fracture strength under axial load.20 upper and lower molars extracted intact were used for the experiment. The teeth were prepared with 2 types of cavities and then divided into 4 groups: 1) 5 mm deep MOD cavities with residual interaxial dentin restored without Ribbond; 2) 5 mm deep MOD cavities with residual interaxial dentin restored with Ribbond; 3) 5 mm deep MOD cavities without residual interaxial dentin, restored without Ribbond; 4) 5 mm deep MOD cavities without residual interaxial dentin restored with Ribbond. The restored teeth were then subjected to thermal cycling and their fracture strength was evaluated using an Instron device. The Mann-Whitney statistical test was used to compare fracture strength among groups. Finally, a descriptive analysis of the verified fractures was performed.There was a statistically significant difference between groups 1 and 2 (P = 0.0090) in the loading force required for a fracture. In contrast, there was no statistically significant difference between groups 3 and 4 (P = 0.7540). Groups 1 and 2 had the fewest non-restorable fractures, in contrast to groups 3 and 4.Ribbond fiber application in MOD cavities seems to be more effective in terms of strengthening where cavities have interaxial dentinal tissue. Key words:Ribbond fibers, fracture strenght, direct dental restorations.