A new polymer nanocomposite consisting of silicon nitride (Si3N4) nanoparticle as a conductive filler and poly (methyl methacrylate) (PMMA) as a polymer matrix was synthesized using "click" chemistry. The synthesis of this nanocomposite is a three-step procedure including (i) the preparation of silicon nitride nanosheet (SNNS) by mechanical exfoliation, (ii) the modification of SNNS with successive chemical modification processes, (iii) performing click reaction between alkyne functionalized PMMA and azide functional SNNS. Characterization results confirmed the functionalization of SNNS surface and the successful "grafting to" method using click reaction. Furthermore, the new thermal conductive nanocomposite was formulated as a thermal interface material (TIM) and its performance as a TIM material was investigated using Thermal Conductivity Analysis (DTC). It has been found that the thermal conductivity of the chemically modified nanocomposite (PMMA-SNNS) based TIM is considerably higher than that of TIM prepared with physically blended nanocomposite (PMMA+SNNS) having the same composition.