Leukemic cells remaining in the body is the main problem for cancer patients, and these cells are called Leukemic Stem Cells (LSCs). Many studies have revealed that the overexpression of the Wilms' tumor 1 (WT1) protein is related to leukemogenesis. Curcuminoids (curcumin, demethoxycurcumin, and bisdemethoxycurcumin) from Thai turmeric (Curcuma longa Linn.) are the focus of this study because they have been previously been reported to show potent antileukemic activity. This study aims to investigate the cytotoxic effect of in-house curcuminoids on the human leukemic stem cell line (KG-1a) when compared to other leukemic cell lines (KG-1 and K562). MTT assays were used to determine the cytotoxicity of curcuminoids at various concentrations. Curcumin exhibited the strongest cytotoxic activity on KG-1a cells with IC50 values of 13.6 ± 1.9 µM. To determine the effect of curcuminoids on WT1 and CD34 protein expressions by Western blotting, KG-1a cells were treated with noncytotoxic concentrations (IC20 value). Bisdemethoxycurcumin showed the strongest suppression of WT1 and CD34 protein expressions with 73.3 ± 1.4 and 82.9 ± 2.0%, respectively. In summary, curcuminoids, especially bisdemethoxycurcumin, could inhibit WT1 and CD34 protein expressions. Thus, bisdemethoxycurcumin is a compound that calls for further studies of its process in the inhibition of WT1 and CD34 expressions in LSCs for the leukemia treatment.