This study investigated the influence of iron concentration on the thermodynamic properties of syndiotactic polypropylene (sPP)/iron composites in the melt phase in a series of samples with varying iron content, ranging from 0 to 10% with a step of 2%. The HYSYS software was used to estimate molecular weight, critical temperature (Tc), critical volume (Vc), and eccentricity (Ec). The results showed that all these properties changed as the iron content varied. The findings of this study confirmed that iron concentration has an impact on the mechanical and chain dynamic (microscopic) properties of polymer composites.