Indonesia is known as a country with agricultural sector in food producing. Major food such as rice (Oryzae Sativa). As the main commodity and the increasing demands from year to year, rice production is known to decreased due to high pest and plant diseases. This research is focusing on designing an expert system to diagnosis the diseases of rice plants with bayes' theorem. The Bayesian approach is a powerful statistical procedure in decision support. Expert system in this research is a program that acting as a smart advisor or consultant for equipped the farmers with sufficient knowledge related with plant conditions. Total 19 diseases and 25 symptoms found for data of rice plant diseases and the design of the systems used waterfall approach which start from analysis, system design, coding and testing. This research also using manual calculation of probability as example. As the result, an expert system is design to support farmers in rice diseases detection.