Here, we report an incidental observation obtained by an animal-borne video logger of courtship behavior by an adult male green turtle (Chelonia mydas) directed toward an immature green turtle. Our result hypothesized that males may select a mating partner mainly using a visual cue based on size, but without high acuity, so that they can distinguish the size of mature females from that of immature turtles. Hence, male turtles may visualize the objects moving underwater that have the similar size to a mature female turtle as a mating partner, and consequently randomly attempt to court turtles including both immature and mature turtles.