Purpose: This study investigated the cuspal deflection in premolar teeth restored with different bulk-fill resin composites with or without using a liner. Materials and Methods: 60 extracted human upper premolar teeth were used. The samples in this study were divided into three main groups (20 samples for each) according to the materials used; Each group was subdivided into two subgroups according to presence of liner. Standardized large MOD cavities were prepared. The distance between two reference points on cusp tips was measured before the restoration using a universal microscope. Then the teeth were restored with different bulk-fill resin composites (Filtek bulk fill, X-tra fil bulk fill and Activa Bioactive Restorative Material) with and without a liner (Activa Bioactive base/ liner). Second reading of cuspal deflection was measured 15 min after the completion of restorations. All samples were subjected to thermal cycling using thermal cycler machine. Then the third reading of cuspal deflection was measured. The data were statistically analyzed with Kolmogorov-Smirnov and Shapiro-Wilk tests. Results: no statistically significant difference was found between the mean cuspal deflection of the three composites and cuspal deflection without liner showed statistically significantly higher mean value than with liner. Conclusion: The placement of Activa Bioactive base/liner beneath the bulk-fill resin composite restorations decreased cuspal deflection.