Quality of the polymer product produced by injection molding process are strongly affected by the mechanical properties of the skin-core structure formed during injection molding operation. It is desirable to know the mechanisms of the skin-core structure and the influence from process parameters and material properties to the skin- core formation in the injection molding. This paper focuses on characterisation of the hardness, modulues, elastic and viscoelastic properties in the skin-core structure of the HDPE polymers by using nanoindentation approach in order to understand the formation of mechanism of skin-core structure, and to facilitate design and dimensioning the polymer part, optimize molding process. The distribution of hardness and modulus were also measured with grid mapping technique by using sharp berkovich indenter.