This study intends to identify applicability of natural dye extracted from Aplysia kurodai, aiming to standardization of Aplysia kurodai natural dyeing through scientific validation by analyzing characteristics of pigment elements to review dyeability, colorfastness and antibacterial activity. Such material as silk fabrics that is animal fiber were used for this purpose, and derived following summarized results. UV/VIS spectrum analysis on the pigment of Aplysia kurodai that was extracted from purple gland showed that maximum absorbtion was near 540 nm. The silk fabric optimal dyeing can be achieved at a temperature of $55^{circ}C$ with a colorant concentration of 5%. Dyed at $55^{circ}C$ with interval of 10~90 minutes for identifying dye uptake over time to observed slow increase of dye uptake over time, and equilibrium occurred at 50 minutes. For dye uptake according to pH, while dye uptake was superior in acidity, it decreased rapidly in a base after pH 7. For color changes according to pH variation, it was reddish purple in acidity and was purplish red in a base. For color changes according to mordanting method, more clear color change had been obtained when process with aluminium pre-mordanting than non-mordanting and post-mordanting. The colorfastness to light, perspiration and washing was 1, 4~5, and 3~4 ratings respectively. The silk fabric dyed with Aplysia kurodai demonstrated excellent antimicrobial activity to Staphylococcus aureus and Klebsiella pneumoniae. The Aplysia kurodai can be used as a new colorant for the natural dyeing of silk.