Proteasome inhibitors can improve the efficiency of cancer treatments by inhibiting nuclear factor κB(NF-κB) activation in cancer cells. Lentils are a type of beans of which consumption of such beans is increasing. The purpose of this study was to investigate the effects of lentils extract (LE) on the proteasomal activities, NF-κB activation, and cell cycle in HepG2 human liver cancer cells. LE treatments inhibited proteasomal activities at concentrations of 10, 50, and 100 μg/mL respectively, and repressed NF-κB activation at concentrations of 1, 10, and 100 μg/mL respectively, in HepG2 cells. LE treatments at concentrations of 1, 10, and 100 μg/mL respectively, increased sub-G1 cell population in HepG2 cells, which may be the result of apoptosis. The results suggest that LE inhibited NF-κB activation partially with its proteasome inhibitory activities, and the increase of sub-G1 cell population was induced partially, by inhibition of NF-κB activation in HepG2 cells.