This research aims to understand the tourists’ behaviors and the marketing mix towards “Discover Thainess” for tourism development guidelines in the Northern Region of Thailand. Data was collected from questionnaires, both in Thai and English. The samples were selected by Convenience or Accidental Sampling from 400 tourists who come to travel in the Northern Region of Thailand, both Thai and foreign tourists. The research result found that most of the tourists have never been to visit Thailand for the campaign “Discover Thainess”. The tourism purposes are mostly for recreation by traveling with the family. The cost of travel for 1 person each time is more than 3,000 Thai Baht. The perception of “Discover Thainess” of tourists in terms of the image of the tourist attraction (x̄ = 4.57, SD.= 0.26) and the uniqueness of the tourist attraction (X̄ = 4.54, SD.= 0.18) are at the highest level. The result also shows that most of the tourists have no knowledge about the campaign of “Discover Thainess”. The factors of marketing mix affect tourists’ decision to travel under the campaign of “Discover Thainess” in overall in good level (X̄ = 4.13, SD.= 0.13). The tourism development guidelines under the campaign of “Discover Thainess” in the Northern Region of Thailand should focus on the following: 1) Preserve the culture, local tradition and lifestyle in the context area by encouraging variety form of tourism but still keep the unique identity of the area. 2) Establish the continuity of the natural resources and culture. 3.) Concentrate on the quality of the environment and experience in the recreational the tourists will get. 4.) Create the balance between the needs of the tourism industry and the needs of “Discover Thainess”. 5.) Create the social of culture, knowledge, local wisdom, public consciousness and develop the potential of “Thainess”. 6.) Develop the visitor economy to be balance and sustainability. And 7.) Manage the natural resources, energy, and the environment as the base for sustainable development.