The present study was intended to comprehensively review the guideline for the protocol and important considerantions for the performance of clinical trial of drugs. In this review, German Drug Act(Arzneimittelgesetz), GCP Guideline of the European Union, varoius publications and my own experiences were refered. The most important points are that clinical trials should be performed ethically as well as scientifically. Therefore, trial protocols should be prepared to fulfill this respects. Moreover, the protocols should be feasible. This means the protocol should be prepared in consideration of the manpowers and facilities. The protocol should be placed on the forcus on primary parameters. The number of subjects should be enough to evaulate significant differences. Investigators should remember that people on the street are not suitable as volunteer for clinical trials. Compliance, sex, age, pharmacogenetics, body mass index, medication, anamnesis and the results of biochemical screening test of volunteers should be considered to select subjects. Foods, physical and psychological stresses are also important factors during clinical trials. Informed consent should be written easily and lay, should inform to subjects the objectives of trial, information of trial drugs, risks and profits from participation of clinical trials.