This paper presents a computer program for easy and rapid disease diagnosis of olive flounder diseases. To design the program, standard diagnosis process of the 14 olive flounder diseases was first setup, then implemented four-steps diagnosis program. To run program, first input fundamental information such as water temperature, size of the diseased fish. Then sequentially, three categories of key factors for disease diagnosis which include external clinical signs, internal clinical signs and microscopic observations are selected. When a user selects the observed signs of olive flounder from the listed options, the program provides maximum 5 presumed disease candidates in order. The disease information, treatment and prevention methods are provided by connected web server through internet. The program would support fish doctors and farmers by providing easy and rapid diagnosis of diseased olive flounder.