The biocomposites of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3,4HB)] and recycled carbon fiber (RCF) were prepared via a melting extrusion. The crystallization behaviors and kinetics of the P(3,4HB) matrix in composites were exclusively studied under both nonisothermal and isothermal conditions. The corresponding results indicated that the P(3,4HB) either neat or compositing with RCF had a dual-peak cold crystallization behavior in the nonisothermal condition, and the isothermal crystallization rate of P(3,4HB) in composites was jointly determined by the rates of the nucleation and the crystal growth and integrity. However, it was reduced with the incorporation of RCF. Wide-angle X-ray scattering investigation demonstrated that the presence of RCF did not change the crystallization mechanism and crystalline structure of the P(3,4HB) matrix, but the crystallinity of the P(3,4HB) either neat or compositing with RCF was enhanced with an increase of crystallization temperature. Dynamic mechanical analysis revealed that the storage moduli of P(3,4HB)-based composites were significantly improved with increasing the RCF loadings, and the dual internal friction peaks corresponding to the thermal motion of surface molecule of crystalline zone and the glass transfer of whole macromolecules were observed on the thermograms of the P(3,4HB) either neat or compositing with RCF. The mechanical properties including tensile, flexural, and notched Izod impact strength were significantly improved in the presence of RCF, and such reinforcing and toughening effects were due to the good interfacial adhesion between RCF and P(3,4HB) as a result of bonding effect of silane coupling agent. Scanning electronic microscopy further confirmed a good dispersion of RCF in P(3,4HB) matrix and a strong interfacial interaction between fibers and matrix.