Biobased non-fossil polyester poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P3/4HB) containing 4.0 mol % 4-hydroxybutyrate (4HB) was melt-mixed with short glass fibers (SGF) via a co-rotating twin-screw extruder. The compositing conditions, average glass fiber length and distribution, thermal, crystallization, and mechanical properties of the P3/4HB/SGF composites were investigated. Calcium stearate, two kinds of paraffin wax and modified ethylene bis-stearamide (TAF) were investigated as lubricants for the P3/4HB/SGF composites. It revealed that TAF is the most efficient lubricant of the P3/4HB/SGF composites. Coupling agents 2,2'-(1,3-phenylene)bis-2-oxazoline (1,3-PBO) and pyromellitic dianhydride (PMDA) were used as end-group crosslinkers to reduce the degradation of P3/4HB and increase the mechanical properties of the P3/4HB/SGF composites. It showed that 1,3-PBO is the efficient coupling agent. The optimum condition of the P3/4HB/SGF composites is 1.5 phr TAF, 1.0 phr 1,3-PBO, and 30 wt % glass fiber content. And the maximum of tensile strength, tensile modulus, and impact strength of the composites is 3.7, 6.6, 1.8 times of the neat P3/4HB polymer, respectively. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012