<p>A sheath/core electrospinning system was employed to fabricate luminescent poly(L-lactic acid) (PLLA) electrospun nanofiber yarns as the sheath, with microfilament poly(ethylene terephthalate) (PET) yarns serving as the core. To achieve this, varying amounts of strontium aluminate (SrAl₂O₄: Eu²⁺, Dy³⁺) phosphorescent particles (SAOED) were incorporated into the PLLA matrix. Additionally, (PLLA–SAOED)/PET electrospun nanofiber yarns with different twist levels were also produced. The morphology, crystalline structure, thermal behavior, tensile properties, and luminescent characteristics of the resulting yarns were systematically investigated. SEM images showed that average nanofiber diameter decreased from (661.26 ± 96.10) nm for pure PLLA electrospun nanofiber yarns to (419.18 ± 65.39) nm for PLLA containing 5% SAOED. Correspondingly, the overall yarn diameter also decreased as the SAOED concentration increased, showing a maximum 37% reduction. Thermal analysis revealed that increasing the SAOED content had negligible effects on the thermal properties of the fibers. Tensile tests demonstrated that the incorporation of SAOED particles did not significantly compromise the tensile properties of the (PLLA–SAOED)/PET yarns, with values comparable to those of the non-loaded samples. Moreover, Upon UV light exposure, all luminescent yarn samples emitted a strong green phosphorescent band. The afterglow intensity of the yarns was significantly influenced by both SAOED content and twist level, with higher luminescence observed at increased SAOED loading and lower twist levels. These results suggest that luminescent PLLA nanofiber yarns can be successfully fabricated via sheath/core electrospinning strategy without sacrificing mechanical integrity, highlighting their potential for applications in biomedical engineering, smart textiles, and other advanced functional materials.</p>