This study was designed to investigate the correlation between the degree of knowledge in regards to the importance of oral health and the smile scores as well as the correlation between their actual oral conditions and smile scores. The author surveyed the knowledge and the actual oral health condition of the subjects, 150 high school students (80 males and 70 females), and their frontal pictures were taken when they had full smiles. After evaluation of the smiles of the subjects by five dentists, the results were analysed. This study showed that there was neither a correlation between the smile score and the knowledge of oral health nor the actual oral condition. The investigation showed that the actual oral condition and care was performed well even if there were some wrong answers in the questionnaire, which asked about proper oral health. In conclusion, the smile could be related to the repetition of the habit of smiling rather than being related to the knowledge of oral health or their actual oral condition.