Based on the structural properties of cholesterol and lipid molecules and on some experimental studies concerning the effect of cholesterol on the lipid bilayer phase transition, a model for lipid-choIesterol mixtures is proposed. The scaled particle theory for mixtures of hard disks is used to describle the hard core interactions among the lipid, and lipid-cholesterol molecules. Together with a semiempirical potential to describe the long range interactions, we find that the model provides accurate description of the manner in which cholesterol disrupt lipid phase transition. This indicates that the excluded volume effect is a major underlying mechanism for lipid-cholesterol mixtures.