We undertook this study to suggest the optimal spinning process conditions that provide a proper range of tenacity and biodegradability in textile fibers. The effect of melt-spinning speed and heat treatment on the mechanical properties and biodegradability of poly(lactic acid) (PLA) fibers were investigated. PLA was spun at a high spinning speed of 2000-4000 m/min, and each specimen was heat-treated. Mechanical properties were estimated by measurement of the breaking stress, and the degree of crystallinity was evaluated with wide-angle Xray scattering. Biodegradability was estimated from the decreases in breaking stress, weight loss, and degree of crystallinity after soil burial. The results of the experiment reveal that heat treatment of the PLA fibers increased the breaking stress and crystallinity. With increasing spinning speed, breaking stress and crystallinity also increased. An increase in spinning speed was more effective than an increase in heat treatment for enhancing the breaking stress within the range of this study. From the soil burial test, it was revealed that an increase in spinning speed and heat treatment decreased the biodegradability of the fibers. X-ray analysis of the soil-buried fibers showed that fibers with higher crystallinities began to degrade more slowly. (c) 2006 Wiley Periodicals, Inc.