The relationship between body length and color of Elysia ornata (Swainson, 1840) was examined by laboratory experiment and field observation. The results indicated that the difference in body color is not an individual variation but is related to growth. Body coloration follows a sequence from transparent, pale green, dark green, greenish white and to beige, with the length reaching maximum size at dark green, and decreasing thereafter. Based on these results, it was inferred that E. ornata has an annual life cycle beginning in late spring.