It is well known that the Internet provides rich resources for the education of teachers and students. Thus, the purpose of this study is to determine the relationship between high school teachers' educational Internet usage self-efficacy belief and Internet addiction. More specifically, such aspects are examined according to age, gender, length of service, and computer proficiency. The participants in this study consisted of 95 teachers in the Fatih Project, which is a pilot high school program based in Kastamonu, Turkey. For the collection of the research data, the Educational Internet Use Self-efficacy Beliefs Scale and the Internet Addiction Scale were employed, after which the data was analyzed by using Pearson's product-moment correlation coefficient and variance analysis. The results indicate that no significant difference exists between the teachers' educational Internet usage self-efficacy belief and Internet addiction, especially in regard to gender and length of service. In addition, although no significant difference can be found between the teachers' competence in the use of technology and Internet addiction, those with greater self-efficacy belief generally scored higher on the Educational Internet Usage Self-efficacy Beliefs Scale. The implication of the findings is that it is necessary to increase the self-efficacy of teachers regarding educational Internet use, which, in turn, can positively impact their students' information skills.