Background: Colorectal cancer is one of most common cancers in women and men and one of the major causes of death due to neoplasia. Colonoscopy is considered as the most accurate diagnostic procedure to detect colorectal cancer at the earlier stages. Objectives: The current study aimed to evaluate the effects of an education program using the Theory of Planned Behavior on promoting behavioral intention among first degree relatives of colorectal cancer patients. Materials and Methods: A quasi-experimental study conducted to evaluate the effectiveness of an educational program to promote attitudinal factors associated with early detection of colorectal cancer in 99 first degree relatives of colorectal cancer patients aged more than 20 years in Yazd city, Iran. A researcher made questionnaire forwhich validity and reliability were confirmed by expert point of view and pilot testing was employed for data collection. Questionnaires were filled in before and after educational intervention. The registered data were transferred to SPSS 19 and analyzed by paired T-test, Man-Whitney and Wilcaxon. Results: Mean scores of knowledge, attitude, perceived behavioral control and intention regarding colorectal cancer increased after education significantly (P<0.05). Conclusions: Application of the Theory of Planned Behavior has positive influence on promoting intention behavior. It is therefore recommended to apply educational programs to promote behavioral intention.