Unscientific disposal of sewage, industrial effluents, and solid waste has contaminated urban soil with heavy metals. Zinc (Zn), even though is a micronutrient, can be considered toxic when in excess quantity. In recent studies, use of nano scale material for remediation of toxic heavy metals is being established. Nano Zero Valent Iron (nZVI) is used here to study the effect of grain size distribution (GSD) on immobilization of excess Zinc in soil. Three different soils, with (10F)10% finer, (50F) 50% finer, and (90F) 90% finer particles than 75 microns were considered for the studies. Immobilization efficiency was evaluated using leachability of Zn obtained by performing toxicity characteristic leaching procedure (TCLP) (USEPA 1311) on all the samples, spiked with Zn, and treated with nZVI. The experimentation involved the treatment of the spiked samples, each featuring three levels of contamination, with varying degrees of nZVI dosage. Variation in Zn speciation was observed through sequential extraction procedure. The experiments were designed and analyzed as per Face Centered Central Composite Design (FCCCD) of Response Surface Methodology (RSM) matrix. Immobilization efficiency was found to be 12% more for coarser soil compared to that of the finer soil for lower contamination and treatment levels.