Cassava starch was blended with glycerol to prepare thermoplastic starch (TPS). TPS was premixed with sericin (TPSS) by solution mixing and then melt-blended with polyethylene grafted maleic anhydride (PEMAH). The effect of sericin on the mechanical properties, morphology, thermal properties, rheology, and reaction mechanism was investigated. The tensile strength and elongation at break of the TPSS10/PEMAH blend were improved to 12.2 MPa and 100.4%, respectively. The TPS/PEMAH morphology presented PEMAH particles (2 µm) dispersed in the TPS matrix, which decreased in size to approximately 200 nm when 5% sericin was used. The melting temperature of PEMAH (121°C) decreased to 111°C because of the small crystal size of the PEMAH phase. The viscosity of TPS/PEMAH increased with increasing sericin content because of the chain extension. Fourier-transform infrared spectroscopy (FTIR) confirmed the reaction between the amino groups of sericin and the maleic anhydride groups of PEMAH. This reaction reduced the interfacial tension between TPS and PEMAH, which improved the compatibility, mechanical properties, and morphology of the blend.