Type 1 diabetes is on the rise worldwide. Although nutrition education for patients with diabetes has become a routine practice, specifics and impacts of such educations need to be more researched. This study examined the status of nutrition knowledge, dietary attitude, and dietary behavior among children and adolescents with type 1 diabetes (9-19 year-old) and explored factors influencing dietary behaviors related to diabetes by applying the Theory of Planned Behavior. Face-to-face interviews, using a pre-tested structured questionnaire, were conducted with 32 participants (11 boys and 21 girls) with type 1 diabetes followed by a diabetes clinic in a university hospital. This study found that the level of nutrition knowledge related to diabetes was generally low at 4 points out of a possible 10, however, the dietary attitude related to diabetes was found to be generally good at 26 points out of a possible 30. Participants were motivated to follow medical staff recommendations the most; however, their family was also important. Perceived behavioral control was low especially for eating-out and portion control. The dietary behavior related to blood glucose control showed low at 13 points out of a possible 20. Regression analysis showed that perceived behavioral control (p < 0.001) was significantly related to the dietary behavior related to blood glucose control. This rare study with children and adolescents with type 1 diabetes showed that nutrition education should include a component to improve perceived behavioral control through high-risk situation management.