In the past, the stress-strain behavior of natural rubber has been studied by a variety of mechanical analytical techniques in order to understand the role of induced crystallization by deformation. But, this was generally done in a sequential manner by stretching to a certain extent and examining the stretched part separately. This type of sequential experiment may not follow the exact dynamics of the process and thus may lead to erroneous conclusions regarding the evolution of microscopic mechanisms. It is therefore advantageous to measure the mechanical response simultaneously with the structural response to map the structural evolution onto the stress-strain behavior. In this paper, simultaneous measurements of the stress-strain behavior and X-ray scattering intensity of vulcanized natural rubber were made continuously by a specially designed instrument during elongation and subsequent retraction. These studies revealed three new characteristics of the strain-induced crystallization. 1. The strain-induced crystallization starts at around 400% strain at 25 degrees C, then increases steeply, although sequential measurements reported that natural rubber starts strain-induced crystallization at 200%. 2. At the onset of retracting, the degree of crystallinity continues to increase, although a drastic decrease of stress occurs. We suggest the morphology of the strain-induced crystallites changes such that do not contribute to stress. 3. Strain-induced crystallites disappear at the strain where the stress-strain loop is closed during retracting. The hysteresis of the stress-strain curve might be attributed to strain-induced crystallization. (C) 2000 Elsevier Science Ltd. All rights reserved.