Dimensional stability is normally defined as the ability of a material to maintain its size and shape under various temperatures and stresses. The parameter, which is used to determine dimensional stability, is thermal expansion coefficient. This work deals with the study of the dimensional stability and thermal stability of high-density polyethylene (HDPE) and its nanocomposites by the presence of different stearic acid (SA) content as a modifier of surface. To investigate the role of surfactant on dimensional stability of HDPE nanocomposite reinforced with nano-sized calcium carbonate (CaCO3), nano-sized calcium carbonates were coated with different SA content. Standard specimens of HDPE/10 vol% CaCO3 nanocomposite were performed using injection method. To study dimensional stability of HDPE/10 vol% nanocomposites, dilatometry method was used. The results showed that addition of nano-sized CaCO3 caused the decrease of thermal expansion coefficient of HDPE. In addition, dimensional stability of HDPE/10 vol% CaCO3 improved due to increasing SA content. Also, addition of inorganic fillers and surfactant retarded the thermal oxidation of HDPE

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