Abstract The industrial application range of welded wood so far has been limited to interior use because of its poor moisture resistance. Influences of some welding and wood parameters such as welding pressure, welding time, and heartwood/sapwood on water resistance of Scots pine (Pinus sylvestris) were investigated. An X-ray Computed Tomography scanner was used to monitor density change in weldlines during water absorption-desorption. Axial samples measuring 200 mm × 20 mm × 20 mm from Scots pine were welded and placed standing in 5-mm-deep tap water. Then they were taken out of the water one at a time and scanned at 10-min intervals until the first crack appeared in the weldline where the two parts of each specimen made connection. Results showed that the X-ray Computed Tomography can be used as an effective tool to study welded wood. Welding pressure, welding time, and heartwood/sapwood showed significant effect on length and location of the crack in the welded zone. Data evaluation showed that combination of 1.3 MPa welding pressure, 1.5 s welding time and using heartwood led to highest moisture resistance, which produced only a very short crack in the beginning of the weldline.