On the basis of our recent findings that licochalcone A isolated from Xin-jiang licorice showed anti-inflammatory and anti-tumorigenic activities, we synthesized more than 40 chalcone derivatives to examine their anti-tumorigenic activities. In vitro inhibitory activity against phosphorylation of phospholipids promoted by tetradecanoylphorbol-13-acetate (TPA) in HeLa cells was adopted as a screening test for anti-tumor-promoting effect. In vivo experimental mouse skin tumors initiated by dimethylbenz[a]-anthracene (DMBA) and promoted by TPA were used to test the anti-tumor-promoting effect of chalcones. In the results, 3'- and 4'-methyl-3-hydroxychalcone showed the highest potency in inhibiting tumorigenesis. They also showed a remarkable inhibitory effect on the proliferation of HGC-27 cells derived from human gastric cancer. We discuss the structure-activity relationship, including stereo-chemical phototransformation, of some chalcone derivatives with reference to their ultraviolet (UV) and nuclear magnetic resonance (NMR) spectroscopic data.