Aim: This study aimed to compare the best restorative approach for the conservative class I cavity by comparing flowable and nanohybrid composites versus the placement technique regarding surface microhardness, porosity, and presence of interface gaps. Materials and methods: Forty human molars were divided into four groups (n = 10). Standardized class I cavities were prepared and restored using one of the following materials: Group I – Flowable composite placed by incremental technique; group II – Flowable composite placed in one increment; group III – Nanohybrid composite placed by incremental technique; and group IV – Nano-hybrid composite placed in one increment. After finishing and polishing, specimens were sectioned into two halves. One section was chosen randomly for the Vickers microhardness (HV) evaluation and the other section was used for the assessment of porosities and interfacial adaptation (IA). Results: The surface microhardness range was 28.5–76.2 (p < 0.05), mean pulpal microhardness range was 27.6–74.4 (p < 0.05). Flowable composites had lower HV than conventional counterparts. The mean pulpal HV of all materials exceeded 80% of occlusal HV. Restorative approaches did not statistically differ in porosities. However, IA percentages were higher in flowable materials compared to nanocomposites. Conclusion: Flowable resin composite materials have lower microhardness than Nanohybrid composites. In small class I cavities, the number of porosities was similar between the different placement techniques and the interfacial gaps were highest in the flowable composites. Clinical significance: The use of nanohybrid resin composite to restore class I cavities will result in better hardness and less interfacial gaps compared to flowable composites.