Biocomposites of potato starch/poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) microparticles were prepared through the solvent casting method. Glycerol was used as a plasticizer. The effects of concentrations of PHBV microparticles as filler and glycerol on crystallinity behavior, surface morphology, dynamic mechanical properties, and thermal stability were studied. Humidity absorption and the water vapor transmission rate (WVTR) were investigated as well. Wide angle X-ray scattering (WAXS) patterns revealed that the plasticizing process occurred successfully. Scanning electron microscopy (SEM) micrographs exhibited good homogeneity of the surfaces for the biocomposites with a lower glycerol concentration. Dynamic mechanical analysis (DMA) results confirmed the reinforcing effect of PHBV microparticles inside the matrix. Thermogravimetric analysis (TGA) indicated that the presence of PHBV microparticles increased the thermal stability of the starch. Results of humidity absorption tests showed that the high hydrophilicity of the starch was reduced once the PHBV microparticles had been incorporated. Also, increasing PHBV microparticles reduced the water vapor transmission rate. However, samples with reduced glycerol content absorbed less humidity and showed a lower water vapor transmission rate.