Corona, which is a phenomenon that occurs when the air around the conductor is ionized can occur in a 20 kV cubicle and damage the equipment. To prevent this occurrence, the air condition inside the cubicle need to be controlled at a safe level. In this study, we maintain an ideal air condition with Arduino-based humidity and temperature control system using ATMEGA328 microcontroller. This system was installed in the 20 kV cubicle at Lokomboro Micro Hydro Power Plant and then tested for performance. From the test result it was found that the system was able to control the humidity and temperature inside the cubicle at 45 % and 26 0C, respectively. By controlling humidity and temperature values at ideal condition, the corona appearance that damage the equipment has been prevented. This installation showed operational, financial, and company image benefit for PT. PLN since it has prevented loss due to damage of equipment and also maintained electrical power reliability.