Xylan deposition and lignification processes were examined in tension wood fibers with gelatinous layers (G-layers) in Mallotus japonicus (Euphorbiaceae). The cell walls consisted of a multi-layered structure of S 1 + S 2 + G + n (L + G), where n indicates the number of repetitions ( n = 0–3) and L indicates very thin lignified layers. The formation and lignification processes of the multi-layered structure of tension wood fibers were examined by light microscopy, ultraviolet microscopy, and transmission electron microscopy (TEM) following KMnO 4 staining. The deposition of xylan was examined by immunoelectron microscopy with a monoclonal antibody (LM11). Immunolabelling of xylan appeared in lignified cell wall layers, except in the compound middle lamella (CML), i.e., the S 1 , S 2 , and L layers but not the G-layers. The density of LM11 xylan immunogold labeling in S 2 layers increased during the formation of G-layers. This increase was due to the shrinkage of S 2 layers during development rather than intrusive deposition of xylan through G-layers. Lignification of the CML, S 1 , and S 2 layers proceeded during G-layer formation. The shrinkage of S 2 layers occurred almost simultaneously with the lignification of the S 2 layers during G-layer formation, suggesting that the S 2 layers shrank with lignification.