Recently the suits for medical malpractice are gradually increasing in this country. The main purpose of this study is to excavate the most suitable theories about civil liabilities on medical malpractice by radiological technologist. To solve the above-mentioned problems in medical malpractice, I have proceeded to make a survey of traditional theories and tried to exvacate the most suitable theories for our medical circumstances among those theories. Both domestic and foreign relevant professional literatures and legal cases were investigated in this study. Several important findings of this study are as follows. First, the nature of legal interrelationship between radiological technologist and physician(or the representative of a hospital) is to define the content of employment. But in the eye of medical law, the interrelationship between radiological technologist and physician is written that radiological technologist should be directed by physician. Second, the nature of legal interrlationship between patient and physician(or the representative of a hospital) is to define the content of legal obligation of physician(or the representative of a hospital), and radiological technoligst execute his obligation as proxy for physician. Therefore, patient can not clame any legal right to radiological technologist. Third, radiological technologist has the obligation of Due Care in medical practice. Fourth, on the medical malpractice by radiological technologist the civil liability can be treated as either tortious liability or contractual liability, and physician(or the representative of hospital) take the responsibility for the damage compensation. In this case, physician has the right of indemnity to radiological technologist. But it should be dinied or extremely limited.