Lignin, a natural aromatic polymer, is a promising candidate for sustainable photonic materials. However, its heterogeneity hinders uniform nanoparticle production. This study employs membrane ultrafiltration to fractionate alkaline lignin into five molecular weight fractions (UL1-UL5) and synthesizes lignin nanoparticles (LNPs) via antisolvent self-assembly. Low-molecular-weight fractions yielded highly uniform, monodisperse LNPs (PDI <
0.1), while higher-molecular-weight fractions produced irregular particles. Notably, a strong correlation between lignin molecular weight and nanoparticle size was observed, with particle size decreasing as the molecular weight increased. Atomic force microscopy and density functional theory simulations provided insights into the intermolecular interactions of lignin fractions, showing that low-molecular-weight lignin exhibited stronger intermolecular forces, facilitating ordered self-assembly. These findings underscore the pivotal role of ultrafiltration in tailoring lignin properties and achieving precise control over nanoparticle formation. This study highlights the potential of ultrafiltration-based approaches for producing sustainable lignin-based photonic materials with customizable optical properties.