Mechanical anisotropy is considered as one of the major drawbacks for materials 3D printed by fused deposition modelling (FDM) technique. The mechanical anisotropy of a variety of 3D printed thermoplastic polymers was investigated before, however, anisotropic characteristics of high impact polystyrene (HIPS) remained unclear. HIPS is impact-resistant, eco-friendly, cost-effective, and most importantly, safe material to use in industry, especially in toys and foods industries. In this study, the HIPS polymer is firstly extruded as filament materials and then 3D printed via the FDM technique for additive manufacturing. Mechanical properties of HIPS samples are measured in different building orientations to study its mechanical anisotropy behaviours. Mechanical properties of 3D printed HIPS are compared with mould injected HIPS as well as 3D-printed acrylonitrile butadiene styrene (ABS) materials. Scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are used to investigate influences of morphology and thermal properties on anisotropic behaviour of 3D printed materials.