The primary objective of this study was the investigation of thermo-mechanical behavior of cellulosic fiber reinforced polylactid (PLA) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) biopolymers. Both PLA and PHBV were processed with 30 wt % of cellulosic fibers; moreover, to improve the processability and mechanical performance, PHBV was previously blended with 30% by weight poly(butylene adipate-co-butylene terephthalate) (PBAT). Secondary target was the comparison of the obtained results to natural fiber reinforced polypropylene (PP) composites reinforced with exact the same fibers and processed by using identical techniques. For validation the thermo-mechanical properties, a dynamic mechanical thermal analysis (DMTA) was applied. Storage modulus (E), loss modulus (E), and loss factor (tan ) were determined. The DMTA results indicate decreased polymer chain motion with resulting improvement of stiffness expressed by the storage modulus. Finally, the effectiveness of fiber on the moduli was investigated. The C coefficient differs in dependence on fiber type, use of coupling agent, and the reference temperature in glassy state. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3175-3183, 2013