In this experiment, sixteen Betta splendens (both intruders and territory holders) were obtained from a local fish shop. Eight will be territory holders and the other eight will be intruders. The Betta splendens will be isolated in their own individual water tank with opaque covers surrounding them in order to prevent them from being agitated due to the sight of other Betta splendens. The aggressive behavior is measured by measuring four characteristics: Operculum (Gill Cover) flair, Ventral (Pelvic) Fin flair, Caudal (Tail) Fin flair, and rams against the fish tank 2a. Construction of the Experimental Groups Sixteen clear 1.5 L fish tanks were obtained from a local fish shop. The fish tanks were split into two different groups: the intruder and the territory holders. All fish tanks were filled with 700 mL of water. The territory holders were grouped into four separate groups with two fish in each group. One group will be the control group without any caffeine added, another is the low dose (120 µg/L of caffeine), the medium dose (200 µg/L of caffeine), and the high dose (280 µg/L caffeine of caffeine). On the other hand, all eight intruders were placed in water tanks without any added caffeine. All fishes were fed regularly in intervals of 8 hours with fish food.