Objective: To explore the effect of microRNA-20a targeted silence on the expression of MICA in breast cancer MCF7 cells and to investigate its effect on MCF7 cells sensitivity to natural killer (NK) cells cytotoxicity. Methods: MCF7 were transfected with microRNA-20a inhibitor using Lipofectamin2000，Real-time-PCR was applied to evaluate the transfection efficiency by detecting the expression of microRNA and Western-blotting was used to detect the expression of MICA. Then the susceptivity of MCF7 to the cytotoxicity of NK cells was assessed based on the crystal violet methods. Results: MicroRNA-20a targeted silencing resulted in the markedly decrease of microRNA-20a. The MCF7 cell up-regulated the expression of MICA and its susceptivity to the cytotoxicity of NK cells was increased significantly after the microRNA silence (P<0.01). Moreover, the up-regulated death rate of MCF7 was abolished by MICA antibody blocking peptide. Conclusions: MicroRNA-20a targeted silencing allows the increased expression of MICA, an important ligand for one of the activation receptors of NK cells. It implicates that downmodulation of microRNA-20a could serve as a target to increasing the susceptivity of breast cancer cells to the cytotoxicity of NKcells.