ZSM-5 zeolites attract considerable attention owing to their wide applications in catalysis and separation. The crystals that are synthesized with tetrapropylammonium ions (TPA⁺) as template have aluminum-zoning, i.e. aluminum being concentrated in the rim part of a crystal. Here, we study the aluminum distribution within individual crystals as function of synthesis time. Crystals with inhomogeneous aluminum distribution persist since their emergence and the degrees of aluminum-zoning in crystals increase with synthesis time. Both the gradual aluminum migration toward crystal surface and the addition of high-aluminum-containing species at late stages of synthesis contribute to the inhomogeneous aluminum distribution within a crystal. As a result, the finally formed crystals have not only the largest crystal size but also the highest degree of aluminum-zoning. The insight into the origin of aluminum-zoning that our work provides advances our understanding of the relationship between aluminum distribution in zeolites and synthesis time to design better catalysts.