This study aimed to investigate the effects of mangosteen peel fractional extracts on the cytotoxicity, Wilms' tumor 1 (WT1) mRNA and WT1 protein levels of four leukemic cell lines (K562, U937, Molt4, and HL60) using MTT assay, reverse transcriptase polymerase chain reaction (RT-PCR) and quantitative RT-PCR (real time RT-PCR) techniques, and Western blot analysis in those four leukemic cell lines. The results showed that ethyl acetate fraction of mangosteen peel extract gave the best cytotoxicity to leukemic cells and also decreased rate of cell proliferation in K562 cells. However, the ethanolic fraction extract showed the best result in Molt4 and U937 cells, and the butanol fraction extract showed the best result in HL60 cells when compared to vehicle control. The WT1 gene expression inhibit in all mangosteen active fractions at non-toxic dose (Inhibitory concentration at 20% growth; IC20) in four cell lines and showed that all the active fractions decreased WT1 mRNA levels in a dose and time dependent manner.