Effect of prolonged natural aging in the laboratory conditions at room temperature during 40 years on the mechanical properties of six different marks of high impact polystyrene (HIPS) was investigated. The deformation curves in tensile experiments were obtained. These curves indicated that the hardening effect is observed for all marks of HIPS after aging. The HIPS were also tested for creep at room temperature and constant tensile force. Specimens of most marks of HIPS does not fractured during 30 days. After natural aging during 40 years, the specimens were fractured in a time-to-fracture range from 10 minutes to 30 hours depending on the mark of HIPS. A modified version of the Maxwell equation written in the effective time scale is used to describe the obtained experimental creep curves. A good agreement between theoretical and experimental creep curves is observed.