Dry ashing and wet digestion are common methods for determining the content and isotopic composition of elements in organic samples. Wet digestion has been used mostly to dissolved samples for determining Cu, Zn and Fe isotopes. Relative to wet digestion, dry ashing is not only more efficient in sample preparation, but also uses less acid, thus suitable for bulk treatment. The results show that:1) for the elsholtzia splendens roots, the process of dry ashing does not cause isotope fractionation of Cu, Zn and Fe; 2) for Elsholtzia splendens stems, the process of dry ashing does cause isotope fractionation of Fe, but no fractionation of Cu and Zn isotopes occurs; and 3) for the Elsholtzia splendens leaves, the process of dry ashing does cause isotope fractionation of Cu and Fe, but no fractionation of Zn isotopes occurs.