Cross laminated timber (CLT) are composed of longitudinal layers and cross layers. And as the laminations of the adjoin layers are glued orthogonally deterioration of the laminations such as check and shake and delamination can be rather frequently come out when CLT is exposed to humid and dry cyclic climate. In Japan more than 100 CLT buildings have been constructed and the number of CLT building is increasing. In some buildings the CLT is designed as to be the exterior members and exposed to the outdoor climate. Several CLT buildings that have CLT as exterior members was investigated to make clear what was ongoing on the CLT panels exposed to outdoor climate. Remarkable check and shake, warp and delamination was observed for the CLT in some of the investigated buildings. To bring out the mechanism of the distortion of CLT under humid and dry cyclic climate and to give a solution to prevent deterioration of CLT a series of laboratory tests were conducted. The CLT panels were set in a climate chamber and exposed to a humid and dry cyclic climate. And the moisture content and the movement of the laminations of CLT were measured. The movement was affected by the species of the laminations and also by the direction of the surface, sap-side or heart-side. And the film forming type painting worked well as to prevent the deterioration and the penetrating type painting could slightly reduce the deterioration.