This work consists in determining the mechanical behavior of bark of sunflower stem. Specimens were extracted at different locations in the stems and were conditioned in chambers of different relative humidity (RH) beforehand. In order to investigate the moisture variation within specimens during specimen’s installation to testing machine, moisture diffusion coefficients of specimens were deduced assuming Fick’s second law prior to mechanical tests. Results show that there was no significant moisture variation within specimens during specimen’s installation. Tensile tests have been performed on specimens. Results show that greater Young’s modulus was obtained for specimens extracted at higher location and conditioned at lower RH.