The anti-tumor peptide M1-21 monomer was used to create nanoparticles（Nano-M1-21，about 50 nm）by the desolution method. The characteristics of Nano-M1-21 were determined by the dynamic optical disper? sion（DLS）and the low pressure transmission electron microscopy（TEM）. The stability of Nano-M1-21 particles was measured in vitro at 37 degrees centigrade. To verify the anti-tumor effects of Nano-M1-21，we used breast can? cer MDA-MB-231，MCF-7 and 4T1 cells to show that Nano-M1-21 was easily absorbed by the cells and its tumor inhibitory concentration was lower than that of monomer peptide. With a mouse breast cancer 4T1 cell-grafting tumor model，we found that Nano-M1-21 showed a good suppressive effect on the tumors. In conclusion，compared to the monomer peptide M1-21，the optimized Nano-M1-21 nanoparticles provide benefits to improve the cellular uptake. and anti-tumor effects