The structural change of the lamellar lipid membrane dehydrated from the swelling L alpha-phase was studied by X-ray diffraction technique. The temperature and the water content of lipid membrane were controlled in a temperature-humidity chamber for Xray in-situ measurement. The result of the lamellar spacing revealed the existence of a first order phase transition induced by dehydration. Far from phase transition, X-ray diffraction patterns were analyzed, and the following changes in membrane structure caused by dehydration were observed: (1) water layers are thinning; (2) lipid bilayers expand in thickness; (3) lipid molecules in the monolayer of the single bilayer are packed more densely; (4) fluctuating tails of lipid hydrophobic chains shorten. More specifically, from its limiting thickness in full hydration, the lipid bilayer expands its thickness exponentially with water layer thinning in dehydration. A simple geometric model is proposed to explain such bilayer expansion induced by dehydration.