The sufficiency economy philosophy (SEP) has been proven to be the best practical concept for helping a country survive poverty and sustain economic growth. However, establishing a sustainable and practical agriculture strategy based on the sufficiency economy philosophy in the highland region of Thailand is not so simple because of various traditional beliefs among the ethnic groups. The theory of panned behavior (TPB) is a generic fact concerning human behavior and belief, regardless of ethnics. This study adapted this theory to capture and develop a belief model for farmers in the highland region. Data from 375 sampled farmers in the highland communities of the Chiang Mai Province were collected by using a multi-stage sampling technique with a questionnaire and analyzed by applying the method of confirmatory factor analysis (CFA). It was found that having a belief in the goal of the sufficiency economy philosophy was the most influential factor for applying this philosophy to sustainable agriculture. The second and third most influential factors were having a belief in the opinions from a group of respectful or trustworthy persons as well as federal organizations, and a belief in self-efficacy, respectively. These three factors of belief were assessed by 10 key information indicators. The results of the assessment and analysis could help local federal agencies make a strategic plan for enhancing farmers’ beliefs in and realization of the usefulness of the sufficiency economy philosophy in highland agriculture.