A new method based on fluorescence imaging and flow cytometry was developed to investigate the transformation process of Saccharomyces cerevisiae AY. Yeast and fluorescent-labeled plasmid pUC18 were used as models of cells and DNA molecules, respectively. Binding of DNA molecules to yeast cell surfaces was observed. Factors influencing DNA binding to cell surfaces were investigated. It has been found that poly(ethylene glycol) (PEG) could induce DNA binding to yeast surfaces, while Li+ showed a weak effect on the binding. When both Li+ and PEG were used, synergetic effect occurred, resulting in the binding of pUC18 to the surface of more yeast cells compared with that in the presence of PEG or Li+ only. It was also confirmed that heat shock, Li+, and PEG all can increase the permeability of yeast cells. This simple method is helpful for understanding the process of yeast transformation and can be used to investigate the interaction of DNA with cell surfaces.