Aim: The aim of this study is to evaluate In-Vitro, Cuspal deflection and Micro-leakage in primary molars restored with : Nanocomposite, Low shrinkage composite, Hybrid conventional composite, Resin modified glass ionomer Versus Amalgam.Materials and methods: Fifty primary molars extracted for orthodontic purpose had been selected and divided into five equal groups M1, M2, M3, M4 and M5each containing 10 teeth. A standard class II cavities were prepared and restored by Valux Plus™, FiltekTM P90, Filtek™ Supreme Z350 XT, EQUIA Fil, and Amalgam capsules. Cuspal deflection was measured to evaluate changes in materials before and after polymerization with a micrometer gauge of Universal Measuring Microscope at 5 x magnification. Microleakage test was assessed by scoring the degree of dye penetration in the tooth restoration interface.The depth of dye penetration was determined using a stereomicroscope and ranked on a scale of 0 to 4. Results: Cuspal deflection and Microleakage tests in resin-bonded composite restorations was reduced with Silorane restorative system when compared to that restored with conventional hybrid composite, Nanoparticle composite, and RMGIConclusion: Cuspal deflection in resin-bonded composite restorations is reduced with Silorane restorative system when compared to that restored with conventional hybrid composite, Nanoparticle composite, and RMGI.With large Class II cavities in primary second molars restored withSilorane-based composite or conventional hybrid composite,Nanoparticle composite, and RMGI; the Silorane-based composite offered much more less microleakage than other types .Polymerization shrinkage, degree of conversion and water sorption are influencing factors that affect the performance of composite restorative systems.Recommendations: Other cavity designs filled with Silorane-based composite, conventional hybrid composite, Nanoparticle composite, andRMGI should be evaluated.