Abstract The properties of ramie fibers treated by thermal treatments at 200 °C under nitrogen and air atmospheres, including thermal stability, crystalline properties, chemical structure, longitudinal thermal deformation behavior and surface performance, were studied. When the ramie samples had been treated, both the lateral order indices (LOI) and infrared (IR) crystallinity ratios were increased, whereas the crystallinity indices of X-ray diffraction analysis were decreased. The absorbed water had a great impact on the properties of the treated ramie samples, promoting the LOI and the crystallite sizes and reducing IR crystallinity ratios, crystallite d-spacings and crystallinity indices. Oxidation in thermal treatments led to the decrease of crystallite sizes and crystallinity indices. The longitudinal thermal deformation behavior of ramie fibers were further measured by thermo mechanical analyzer. The results showed that 2 treated samples with moisture absorption were of thermal expansion properties, the one without moisture absorption and treated under dried air exhibited thermal shrinkage in the temperature range below 100 °C and thermal expansion above 100 °C, while the one without moisture absorption and treated under dried nitrogen displayed thermal shrinkage in the whole temperature range. These different behaviors were attributed to the diversity of chemical structure and crystalline structure in fiber samples resulted from the absorbed water, oxidative decomposition and thermal degradation. The fracture morphology of ramie fibers/epoxy resin composites was observed by scanning electron microscopy, which disclosed that the compatibility between fibers and resin matrix was enhanced through thermal treatments, and the failure tips of fiber samples without moisture absorption are coarser and looser than those with moisture absorption because of the influence of absorbed water on cellulose structure. It can be seen that the atmospheres of thermal treatments and moisture absorption are the main factors to affect the properties of the treated ramie fibers.