Leukemia is a group of common hematological malignancies, and the expression of Wilms' tumor1 (WT1) gene and its product in leukemic blast cells is very high compared with normal bone marrow cells. This study investigated the effect of turmeric curcuminoids, including curcuminoid mixture, pure curcumin, demethoxycurcumin, and bisdemethoxycurcumin, on WT1 gene expression in leukemic cell lines (K562, U937, HL-60, and Molt4). The cell lines were cultured in the presence of 10 μM of curcuminoid that was not toxic to leukemic cells under our experimental conditions. WT1 mRNA and WT1 protein levels were assessed by reverse transcriptase polymerase chain reaction (RT-PCR) and Western blot analysis, respectively. The curcuminoid derivative which showed strong inhibitory effect on WT1 gene expression in this study was selected for study of its effect on WT1 gene expression using various concentrations which were nontoxic (5, 10, and 15 μM) to leukemic cell lines and incubating up to 3 days. Pure curcumin exhibited a strong inhibitory effect on WT1 mRNA and WT1 protein expression. Moreover, pure curcumin significantly decreased WT1 gene expression in both a concentration-dependent and a time-dependent manner (p < 0.05). In summary, pure curcumin decreased both WT1 protein and WT1 mRNA levels in leukemic cell lines. It may be possible to use pure curcumin as a chemotherapeutic agent in human leukemic cancer.