“Winter-worm and Summer-grass” is one of the most valued traditional Chinese medicines used for the replenishment of body health. It consists of the fungus Cordyceps militaris and its natural host of Hepialus armoricanus. Using the water extract from the fruiting body (fungus) and worm (Bombyx mori pupae, artificial host), we have found that the extract inhibited proliferation and viability of human breast cancer MCF-7 cells in a dose and time dependent manner, with IC₅₀ values at about 20 ug/ml when MCF-7 cells were incubated with the extract for 96 hrs. 2D gel analysis revealed that the extract treatment altered the protein expression profiles of MCF-7, suggesting that the differentially expressed proteins might be responsible for inhibition of MCF-7 cell proliferation and eventually for the cell death. MALDI-TOF mass spectrometry analysis of the differentially expressed proteins were performed and their potential roles in cell cycle will be discussed.