Summary Most offshore structures for oil exploitation are composed of a deck and legs produced as a framework by many welded tubular components. The prediction and control of welding deformation is extremely important for the efficient and economic construction of offshore structures. This paper describes a fundamental investigation of the prediction and control of welding deformation in jacket-type offshore structures. Most studies examining the welding deformation of tubular components (cylindrical drums, cylindrical shells) concern the local deformation occurring during circumferential butt welding. This study investigates the welding deformation which occurs when a tube (brace) is welded on the wall on another tube (chord), as during the welding of T or Y joints, in the actual jacket construction process. The local deformations of the braces are measured near the joints. The influencing factors, such as e.g. brace sizes, groove shapes, welding sequences, heat inputs, etc, affecting the local deformation are also simultaneously investigated. The correlations between the deformation and controlling factors are examined. The results show that the total heat input (obtained by summing the heat inputs per unit length of all welding passes) for each joint most affects the local deformation. Relations between the local deformation, i.e. the axial shrinkage, in-plane rotation, and out-of-plane rotation of the brace, are derived by regression analysis. The mechanisms of deformation increase and and decrease are discussed in detail in relation to the total heat input, welding sequence, etc. In the next report, the welding deformation of the structure as a whole will be simulated in a theoretical analysis using the above relations derived for the local deformation.