The aim of this study is to evaluate the microleakage of preheated bulk fill composites in vitro. In the study, 160 slot cavity prepared in mesial and distals of 80 molar teeth were used. In the gingival of the cavities, Estelite Bulk-Fill Flow (Tokuyama,Tokyo,JAPAN) in the mesial and SDR Posterior Bulk Fill Flowoble Base (Dentsply,DeTrey,GERMANY) in distal are used and the remaining occlusal parts are restorated by Filtek ^TM One Bulk Fill Restorative (3M, StPaul,MN,USA). Composites applied at 4 different temperatures (4̊C, room temperature, 39̊C and 55̊C). After finishing and polishing, specimens were thermocycled and then placed in 0.5% methylene blue for 24 hours. Specimens were sectioned into 2 parts in mesio-distal direction and the mikroleakage values at stereomicroscope(x40) were examined. In the statistical evaluation of the microleakage values of Filtek ^TM One Bulk-Fill Restorative applied at 4 different temperatures on the occlusal surface with One-way ANOVA, it was observed that there was a significant difference between the groups (p=0.02). In the comparison of the microleakage values of Estelite Bulk-Fill Flow applied in the gingival region with the One-way ANOVA test at 4 different temperatures, there was no statistically significant difference between the groups (p> 0.05), while there was a significant difference between the groups using SDR Posterior Bulk-Fill Flowoble Base (p= 0.002). The highest microleakage values were observed at 4 °C in all groups. The results obtained in the study show that heating bulk fill composites before polymerization is effective in reducing microleakage, but long follow-up in vivo studies are required. Research Highlights This research was applied to standard cavities prepared at 4 different temperatures, 39°C and 55°C at room temperature, as soon as the bulk fill composites came out of the refrigerator. Micerium Ena Heat composite heating device was used to heat the composites. Stereomicroscope was used to examine the microleakage values of the samples after dye penetration. The most obvious result in this study was that the highest microleakage value was seen in the samples applied as soon as they were taken out of the refrigerator. Microleakage values decreased with increasing temperature. Within the results of this study, it can be suggested that the composite resins should not be used as soon as they come out of the refrigerator and should be heated before use for less microleakage.