The thermal degradation of jute and flax fibers under temperatures between 170 and 210 degreesC for a maximum of 120 min was studied in detail. This article will analyze the effects of the thermal exposure on mechanical properties (tenacity) as well as on fiber fine structure (degree of polymerization and degree of crystallinity). It was found that temperatures below 170 degreesC only slightly affects fiber properties, while temperatures above 170 degreesC significantly dropped tenacity and degree of polymerization. Because of chain scissions, a slight increase in degree of crystallinity was observed.