In clinical trials, it is important to have well designed case report forms (CRF) in order to obtain good quality of data. If CRF design at the first stage of your study goes wrong, after all efforts have been made, you may find practical difficulties in data analysis at the final stage of your study because of incomplete or wrong information. In this paper, the following rules fundamental to CRF design are introduced: rules in general, rules of format or style, categorized responses, forms with multiple records per subject, and international studies. Investigators are well aware of research goals, however sometimes they do not know how to express it on CRF. If they understand the rules fundamental to CRF design, time for CRF development will be saved and CRF completeness will be enhanced.