Statement of Problem: The control of microleakage presents a challenge for posterior composite restorations. The technique for composite placement and treatment technique may cause some modifications in the microleakage. The results of different studies are varied.
Purpose: This study aimed to evaluate the marginal microleakage in class II P60 resin composite restorations prepared with two restorative techniques and two polymerization systems.
Materials and Method: Standard class II cavity preparations were performed on 56 carries- free extracted molars. Then, the teeth were randomly divided into four groups: 1) Bulk placement and conventional polymerization 2) Incremental placement and conventional polymerization 3) Bulk placement and soft start polymerization 4) Incremental placement and soft start polymerization. All the cavities were restored with single bond and P60 (3M/ESPE) resin composite. After thermocycling, the specimens were immersed in 2% methylene blue solution for 24 hours and then washed. After sectioning, dye penetration was assessed by light stereomicroscopy at 40x magnification and then evaluated for microleakage by Kruskal-Wallis Test and Mann-Whitney tests.
Results: Significant differences were found between the bulk with incremental technique and soft start with conventional polymerization. The highest and lowest rates of microleakage were in bulk conventional and incremental soft start group, respectively. A significant difference was found between groups.  
Conclusion: The use of incremental placement technique and soft start system results in less microleakage.