Springwood and summerwood fibers of a long leaf pine holocellulose pulp were dried under various axial tensile loads to determine the effects of drying under tensile load on the mechanical properties and on the fiber structure as determined by X-ray analysis. Elongation measurements during drying showed that the fibers underwent a sudden extension at the commencement of drying which was independent of the drying load. Drying of fibers under load caused an increase in Young's modulus, tensile strength, work-to-rupture, and crystallite orientation; it caused a decrease in ultimate elongation, and the crystallinity remained the same. In general, the springwood fibers underwent much larger changes than the summerwood. Evidence is presented which indicates that the changes in mechanical properties are due to an increase in crystallite orientation and to a more even distribution of stress among the fibers. One of the more practical implications of this work is the conclusion that the large difference in the directional mechanical properties of paper is partly attributable to the change in fiber mechanical properties caused by drying underload.