The property-structure-processing relationship on the forging of isotactic polypropylene (iPP) has been investigated by applying multiple regression analysis. The analysis showed that the optical and mechanical properties of forged iPP can be described quantitatively by phase structures and the compression draw ratio (CR). For optical clarity, appropriate variables to explain the property are the smectic fraction (X-s), the amorphous fraction (X-a) and CR-1. The increase in the smectic fraction and CR reduced the turbidity (for example, the increase in optical clarity) of the sample. The contribution of smectic phase estimated by the analysis was about one-third of the compression draw, as revealed by CR-1. For mechanical properties, X-s, X-a, and CR1/2 were the appropriate variables. The existence of the smectic phase reduced the tensile mechanical properties. The effect of X-s is 12 times higher in modulus than that in strength.