This aim of this study is to evaluate the fracture resistance of maxillary premolars weakened by extensive MOD cavities and restored with nanohybrid composite or bulk fill RBC “BFRC “ with and without a newly introduced short fiber reinforced bulk fill base “SFRC”. 60 sound premolars were allocated into six groups (n=10). Group 1: Sound teeth “positive control”, Group 2: prepared teeth with MOD cavity preparations without restoration “negative control”, Group 3: premolars with MOD cavity preparation restored incrementally with Nanohybrid RBC (Grandio®, VOCO, Germany). Group 4: premolars with MOD cavity preparation restored by bulk fill RBC (x-tra fil®, VOCO, GERMANY). Group 5: premolars with MOD cavity preparation restored by 2mm short fiber reinforced composite base (Ever X posterior®, GC™) and with 2 mm Nanohybrid composite veneer (Grandio®, VOCO, Germany). Group 6: premolars with MOD cavity preparation restored by 2 mm short fiber reinforced composite base (Ever X posterior®, GC™) and veneered by 2mm bulk fill RBC (x-tra fil®, VOCO, Germany). Load was applied on all specimens using universal testing machine with crosshead speed 1 mm/minute applied on teeth till fracture. The outcome is the load at which the specimen was fractured in Newton. The fracture mode was determined either favorable or unfavorable. Sound teeth (1336.03±105.48) showed statistically significant highest mean fracture resistance (N), followed by Nanohybrid RBC with SFRC base (1225.98±195.15) followed by insignificantly lower values for BFRC with SFRC base (1095±104) followed by Nanohybrid RBC (1053.8±114.99) and BFRC (1028.72±94.68) with an insignificant difference between them; followed by negative control (611.55±93.39) at ≤0.001.