Due to the fact that Photovoltaic (PV) systems must be practically installed in outdoor areas for converting solar radiation into electric power. For this reason, PV systems have to face the risk of damage from lightning. Mostly, PV Plants installed in Thailand, they have still not been installed lightning rods appropriately and engineers do not study lightning protection properly for PV systems. Then this paper has been gathering information about lightning protection in PV systems to develop a computer program for making a decision on either need to install lightning protection in PV system or not., the computer program can give a design on how to install lightning rods by using the protective angle method. This proposed computer program is tested by using the information from an existing 25kWp PV system installed at Thayang district, Phetburi province of Thailand. It was found that the existing 25k Wp PV system there faces a risk of damage from lightning strike on average 4.15 times per year and damage costs could be high. Accordingly, the proposed computer program can correctly design for lightning rod installation and verify the results by another 3D drawing program with the same method. From the results showed that all components of an existing PV system mentioned are properly protected by lightning rod. Consequently, the project owner is recommended that the existing PV system should be protected by installation of lightning rod.