Injection molding is one of the most popular manufacturing processes for producing good finishing plastic products with low cost and high volume production, especially for the production of plastic bottles. In order to produce high quality plastic bottle with specific size, the injection moulding mould need to be properly designed. This study is aimed to design injection moulding mould for producing three different sizes of Polyethylene Terephthalate (PET) parison. The actual dimensions of a commercial bottle preform of parisan of 25g weight were measured. PET was used as thermoplastic material because it has good strength and light weight properties. The designing process involved two primary components; (1) Female section consists of cavity plate as the main component and (2) male section consists of core plate as the main component. The effect of parisan size on the mould design was evaluated. Three different designs of female and male sections were constructed using CATIA software based on 15g, 20g and 30g parisan weight. The designs were also compared to the existing mould system of 25g PET parisan. It was shown that the design of insert cavity of female section and core cavity of male section were highly influenced by the size of the preform.