Antibiotics are chemical substances produced by various species of microorganisms that suppress the growth of other microorganisms or may destroy them. Among the more than 4000 antibiotics that has been identified, about 20s are using as the therapy of infectious fish diseases. There are several methods used to classify and group antibiotics, and the most common classification has been based on chemical structure and proposed mechanism of action. The effect of antibiotics may be determined by the kind of fish pathogens and by the external environment surrounded the infected fish. It implies that the kind of antibiotics and its application method should be decided after the determination of the reasons of fish disease. The uncontrolled usages of antibiotics may induce the selection of resistant mutants appeared spontaneously and present in any group of bacteria. The epidemic spread of such antibiotic resistant strains of fish pathogenic bacteria already has been reported in various districts of japan. Importantly, transferable drug resistant(R) plasmids were detected in strains of most of fish pathogens. Based on those reports, the antimicrobial resistance appears to be a rapidly emerging problem in the fish industry on the country. The expanding literatures on the pharmacokinetics, clinical trials, withdrawal periods and efficacy of environmental effect for the commonly using antibiotics have met the needs of data for the practical application of antibiotics. However, the most important thing for the treatment of fish diseases would be the communication and exchanging of information between the site of aquaculture and the diagnostic laboratory.