Objective: This study is aimed to investigate interfacial micro gaps between bases and hard setting calcium hydroxide liner. Materials and Methods: Twelve sound extracted human maxillary premolars were selected and immersed in 0.1% thymol solution. Samples were subjected to Class I cavity preparations with the width of 2.5 mm buccolingually, 3 mm mesiodistally, and 2 mm depth from the dentinoenamel junction (DEJ). The cavities were lined with hard setting calcium hydroxide lining (Dycal®), (Dentsply, USA) and then divided randomly into two groups. The cavities were restored with smart dentin replacement (SDR®), (Dentsply, Germany) and glass ionomer cement Ketac™ N100 (3M ESPE, USA) for Group 1 and 2, respectively, (n = 6 for each group) up to DEJ level. All samples were then packed with composite resin. Samples were cut longitudinally using a hard tissue cutter (Exact, Japan) and sanded with increasing grit sandpaper (#320, #500, #800, and #1200) for 30 s each and subjected for interfacial micro gaps analysis using scanning electron microscopy. Results: There was a significant difference of micro gap formation between two groups of base materials and hard setting calcium hydroxide (Dycal®) (P < 0.05). Conclusions: Lesser micro gap between Dycal® and SDR® compared to Dycal® and Ketac™ N100 suggested SDR® as a better base material to be used with Dycal® for deep caries management.