The aim of this in vitro study was to evaluate the effect of different packing techniques (incremental and bulk packing) of new bulk fill posterior resin composite materials from the packable type as well as the effect of storage time on the gingival microleakage in class II cavity preparations.Forty extracted upper premolar teeth were collected and the teeth were prepared to standard MOD cavities then randomly divided into two equal main groups according to the resin composite system used (either Xtra-fil® or Tetric N Ceram Bulk Fill®) each was used with the recommended etch and rinse bonding system from the same manufacturer). Each group was divided into two equal subgroups according to the resin composite packing technique either incremental or bulk. Each subgroup was divided into two classes according to time elapsed after resin composite packing either immediate or after three months storage in artificial saliva at room temperature. Gingival microleakage assessment was done by evaluating the penetration of 2% methylene blue dye along the gingival seat and the axial wall of each tooth under 45X magnification Stereomicroscope connected to a computer equipped with the image analysis software program.Immediate post cure degree of conversion (DC) of the two investigated bulk fill resin composite materials were assessed comparing the surface DC to the deep subsurface DC using Fourier transform infrared spectrometer FTIR. A split disc shaped tephlon mold was fabricated with internal hole of 4mm length and 2mm diameter used for making resin composite rods of the same dimension. One millimeter from the external surface of the resin composite rod and 1 mm from the deep surface (i.e. at 4mm depth) were ground into fine powder each was mixed with potassium bromide to form a pellet to be measured.Kruskal Wallis test used to compare between the composite materials, packing technique and follow-up periods within other variables for gingival microleakage. Independent t-test used to compare between different tested Composites within each surface for mean degree of conversion. Dependent t-test used to compare between the top and bottom surface within each material for mean degree of conversion.Results showed that there was no statistically significant difference for the gingival microleakage scores between the bulk packing technique and the incremental packing when tested for both Xtra fil and Tetric N Ceram Bulk Fill at immediate or after 3m artificial aging.Also, there was no statistically significant difference for the gingival microleakage scores between immediate testing and after 3 months artificial aging for both Xtra fil and Tetric N Ceram Bulk Fill when tested either in bulk packing or incremental packing technique.About the degree of conversion results, there was no statistically significant difference between DC results of both external surface and deep surface within the same material Xtra fil while for Tetric N Ceram Bulk Fill there was statistically significant difference between them. Also, there was statistically significant difference between DC results of both external surface and deep surface of Xtra fil and Tetric N Ceram Bulk Fill. ConclusionsUnder the limitation of this study, the following conclusions could be obtained,1.The packing technique either incremental or bulk does not affect the gingival marginal leakage of the investigated bulk fill resin composite materials. 2.Artificial aging has no effect on the marginal leakage of both materials.3.The tested bulk fill resin composite materials showed acceptable degree of conversion that improved the behavior of this material even at the gingival margin.