The work demonstrates the utilization of fractionalized lignin from the black liquor of soda pulping for the development of starch-lignin biocomposites. The effect of ultrafine friction grinding on lignin particle size and properties of the biocomposites was investigated. Microscopic analysis and membrane filtration confirmed the reduction of lignin particle sizes down to micro and nanoparticles during the grinding process. Field Emission Scanning Electron Microscopy confirmed the compatibility between lignin particles and starch in the composites. The composite films were characterized for chemical structure, ultraviolet blocking, mechanical, and thermal properties. Additional grinding steps led to the reduction of large lignin particles and the produced particles were uniform. The formation of 7.7 to 11.3% lignin nanoparticles was confirmed in the two steps of membrane filtration. The highest tensile strain of the biocomposite films were 5.09 MPa, which displays a 40% improvement compared to starch films. Further, thermal stability of the composite films was better than that of starch films. The results from ultraviolet transmission showed that the composite films could act as an ultraviolet barrier in packaging applications. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.