The use of fiber-reinforced composites is increasing in many application areas. Environmental conditions that affect usage in these areas are important to understand the effects of composites on their strength and bonding performance. They also play an effective role in creating more suitable designs. In this study, the behaviors of single lap adhesively bonded composite joint which has different lap lengths and different fiber reinforcement angles are investigated in the hydrothermal environment. To predict these behaviors, an experimental study was carried out at a constant water temperature of 40 oC, 60 oC and 80 oC and variable immersion periods (15, 30 and 45 days). Glass epoxy composite plates with fiber reinforcement angles [0o]8 and [45o/-45o/0o/90o]s were used. Specimens with lap lengths of 25 mm and 35 mm were prepared from these two plates. Absorption rates, failure loads and failure modes were obtained from these specimens placed in hydrothermal environment. These results were compared with each other and with specimens kept at room temperature. In this study; it has been determined that the failure load values obtained from specimens with the fiber sequence [45o/-45o/0o/90o]s are lower than the values obtained from specimens with the fiber sequence [0o]8. The lowest failure load values were obtained from specimens exposed to a temperature of 80 °C. It was determined that when the waiting time of all specimens in the hydrothermal environment and the temperature of the environment increased, the failure loads decreased but the moisture absorption rates increased. Accordingly, in general, in failure modes, fiber failure and adhesive failure were achieved together.