Abstract: How to create a good environment of chicken house is one of the most valuable tasks in modern poultry raising. Pratice indicates that ventilation is an important measurement to control the environment in chicken house. In this paper computer and variable the values of reference volume and characteristic linear length are better taken as those when grains are in initial moisture state． The sum of linear expansion coefficients in 3 perpendicular directions is equal to its volumetric one theoretically． This is confirmed by most published data． The structures of grain endospem， like rice and corn， apear to be isotropic． However published measurement results on linear hydroscopic expansion coefficients show they are anisotropic． What causes whole kernel shrinking anisotropically is still a problem， which needs to be further studied． Suppose that a grain kernel is a twophase system with solid （dry mass） and pure water， and change in the volume of a grain kernel is due to change in the kernel moisture content， a theoretical coefficient of hydroscopic expansion is derived． Since the air phase inside the kernel does not been considered, theoretical coefficient of hydroscopic expansion should be the upper bound for measurement values and this is confirmed by most published data． This property of theoretical coefficient of hydroscopic expansion is useful for experiment determination of the coefficient． Moisture transfer inside grain kernel will cause kernel damage， such as internal voids and micro cracks． Different drying procedures will produce different level of damage． The internal porosity of kernel may be taken as a damage variable which discribes the degree of kernel damage． Damage mechanics will be a useful theory for further investigation of grain stress cracking.