Ubiquitin-proteasome proteolytic system plays an important role in selective protein degradation and regulates cellular events including apoptosis. Cancer cells have been shown to be more sensitive to the proapoptotic effects of proteasome inhibition than normal cells. Thus, proteasome inhibitor can be potential anticancer agent. Since the MeOH extracts of psoraleae semen and processed psoraleae semen showed potent proteasome inhibition activity, the fractions of the extracts were evaluated on the activity to screen the proteasome inhibitors. The $CHCl_3$ fr. of the processed psoraleae semen showed the most potent activity, of which chemical investigation led to two coumarins, psoralen and isopsoralen. Their structures were determined by spectroscopic methods such as $^1H-NMR$ and EIMS spectra.