Licochalcones A (1) and E (2), retrochalcones or reversely constructed chalcones, isolated from the roots of Glycyrrhiza inflate, were evaluated for their cytotoxicities against four different human tumor cell lines; A549 (lung), SK-OV-3 (ovarian), SK-MEL-2 (melanoma) and HCT-15 (colon), using the sulforhodamine B (SRB) assay. The effects of these compounds toward the DNA topoisomerase I (tope I) inhibitory of activity were also measured using the supercoiled DNA unwinding assay. All compounds showed moderate cytotoxicities against the four different human tumor cell lines and inhibited the tope I activity in dose-dependent manners. The inhibition of topo I by licochalcones A (1) and E (2) may explain the cytotoxicities of these compounds against the human tumor cell lines.