Abstract: In situ imaging nanoindentation was used to characterize the differences in longitudinal Modulus of Elasticity(MOE) and hardness of different cell wall layers of softwood tracheids.It was found that the deformation of cell wall was mainly plastic when tips were indented in longitudinal direction.The distribution of longitudinal MOE and hardness along cell wall thickness was uneven with much lower values at the interface between S₃ layer and cell cavity as well as S₁ layer and compound middle lamella(CML) compared with that measured in S₂ layer.There also existed differences in MOE and hardness between S₂ layers of adjacent tracheids.ANOVA indicated significant differences in MOE between secondary wall(SW) and CML,but no significant difference for hardness.Both MOE and hardness of mature wood trachedis were larger than that of juvenile wood,but in a different degree.The SW of mature wood tracheids displayed an MOE nearly 1.4 times of that of juvenile wood,but only 13% harder than juvenile wood.A positive linear correlation between MOE and hardness was also found both for SW and CML of tracheids.