The aim of this study is to evaluate the effect of the addition of zirconium (Zr) as an inoculant to the microstructure and to examine the mechanical properties of test coupons and heavy section cast steel via a conventional sand casting method. The microstructural and mechanical properties of cast steel and Zr-added steel specimens have been studied in detail. Optical and scanning electron microscopes have been used to identify the inclusion morphology and microstructural changes. Tensile and Charpy V-notch impact test was carried out to determine the mechanical properties of test coupons and heavy section cast steel. The results show that the Charpy V-notch impact energy and elongation were improved by adding Zr. Fractographic studies show that the Type III manganese sulfide inclusions located on the dendrite arms evolve to Type I with the addition of Zr.