Palm trees, like other plants, are subject to a variety of diseases during their life. It is causing material and economic losses. So, it would be good to control these common diseases and search for ways that contribute to the correct diagnosis. In this project, we will develop and design an expert system based on mobile application to diagnose diseases of palm trees. This app will serve farmers interested in caring for palm trees in the Arab region, especially Saudi Arabia, by providing appropriate languages to make palm tree diseases easy to diagnose and take advice on. By implementing the certainty factor method. The certainty factor is used as a means of the expert system of action by representing a number that measures the expert’s belief during the diagnosis and symptoms of the disease. Convenient user interfaces, support IOS, Android. This study has shown the ability of the certainty factor method to facilitate farmers’ identification of diseases and pests based on symptoms. We will compile more data on diseases to validate these results in future work.