As a fast-growing commercial wood species, poplar often produces tension wood because of the leaning stems of logs, which has a great impact on the production and utilization. The gelatinous layer (G-layer) was the unique component of tension wood, almost entirely composed of cellulose, which was the main reason for the special performance of the poplar tension wood. This study used the transmission electron microscopy, Raman spectroscopy and nanoindentation to examine the ultrastructure and mechanical properties of every wall layer of the poplar cell wall in order to figure out the ultrastructure and properties of G-layer. In particular, the nanoindentation mapping was combined with manual screening to obtain the mechanical performance data of each layer. The fibers in the tension wood were gelatinous wood fibers with obvious G-layers, and the thickness was 2.53 mu m, which was much larger than that of other wall layers. Compared with the wood fiber in the opposite wood, the S1 layer was thicker and the S2 layer was thinner, and the entire cell wall thickness was increased in tension wood. G-layer in tension wood had a very high cellulose content, and the Raman strength of lignin was weak. In general, the micromechanical properties of poplar tension wood were better, which was consistent with its macromechanical properties. The characteristics of G-layer, including high cellulose content, small microfibril angle and high crystallinity, made G-layer own better elastic modulus (14.08 GPa) and hardness (0.45 GPa) than other wall layers in the tension wood. Moreover, G-layer had a high proportion in the tension wood cell wall, so the average longitudinal elastic modulus and hardness of wood fiber cells in the tension wood were also higher than those in the opposite wood.