Rotational moulding of plastics is predominantly used to produce hollow, single-piece products. Polyethylene in its various types and in powder form is the most commonly used material for this process. To overcome the material limitations, especially when the design parameters are limited as well, researchers have been conducting experiments of incorporating reinforcement and thereby improving the mechanical performance of rotational moulding products. One of the most common problems with reinforcement in rotational moulding is the migration of the filler towards the inside of the mould and agglomerations. In order to find a competitive material which is desirable by the industry, composites of high density polyethylene and different types of glass fibres and carbon fibres were manufactured by rotational moulding. As novel reinforcements for rotational moulding, ultrafine glass fibres and carbon nanotubes were investigated. The influence of low weight fractions of reinforcement on the mechanical performance, tensile, flexural and impact properties of high density polyethylene were investigated.