This paper argues that Ho Wang Lee’s discovery of Hantaan virus as the agent of Epidemic Hemorrhagic Fever(EHF) was the result of repeated failures and trials on new objects and new experimental methods. Generally speaking, scientific papers describe the research process as going a straight way leading to the final results. However, researches in the real world are full of trial-and-errors, misunderstandings, and complex interactions among elements in the laboratory. Ho Wang Lee’s research is a good case to show such characteristics of the scientific research.In this paper, Lee’s researches from 1970 to 1980 are examined into three periods. First, from 1970 to 1973, he studied about EHF using wild rats as an experimental object. Wild rats were widely suspected to have the agent of EHF but he failed to get any results. During the second period from 1974 to 1976, he identified an agent of EHF, Korea Antigen, by introducing a new experimental object, the lung of wild rats, and a new method, Fluorescent Antibody Technique. For last 4 years from 1977 to 1980, Lee clarified its physicochemical features. With the results, the Korea Antigen was registered as the agent of EHF, Hantaan Virus in 1980.