This study was conducted to investigate the gap formation of Bulk-fill resin composite compared to Nanofilled incremental resin composite when subjected to both alkaline and acidic medium at two time intervals at 24 hours and after 3 months. A total number of 156 sound human molar teeth were used in this study. Following cusp tip removal, a box shape occlusal cavity was prepared. Specimens were randomly divided into two main groups according to the material used whether Xtra-fil or Grandio. Specimens were subjected to PH challenge. Microleakage was evaluated with Digital microscope at 40× magnification. Kruskal Wallis test and Mann Whitney test were used to compare between different media, materials and time intervals. The results revealed an insignificant difference between bulk fill and conventional incremental resin composite restorative materials within the three different media and time variables. While concerning the time factor, the results demonstrated there was a statistical significant difference between 24 hours group and 3 months group.