The main factors that causes of dental caries are the host (teeth and saliva), a substrate (food), microorganisms causing caries, and time. The factor that cause of caries is inherent in an individual’s mouth which affects directly to the occurrence of dental caries. Besides due to internal factors, caries can be caused by some external factors. One of them is known about oral health. The knowledge has important role in the formation of a human behavior. The level of this knowledge can provide the understanding on maintaining oral health. Objective: This study aims to understand the relationship of the knowledge of oral health and the severity of dental caries in the children aged 12th-15th. Methods: This is a research study based on analytical observation approached by a cross sectional method using questionnaire and clinical check up. The subjects are 84 students aged 12th to 15th in SMP Muhammadiyah 9 Yogyakarta. This study used stratified random sampling. The severity of caries examined by CSI (Caries Severity Index) and the level of oral health knowledge from the questionnaire. The data are analyzed by Pearson correlation test. Result: From this study, we can see that the level of knowledge of 66 students or 78,5% students is 13-18. While the severity of caries which measured by Caries Severity Index has the most point of 1 in a number of 37 students or 44%. The result of statistical test used Pearson correlation test is known by a score of p-value, 0,109 which is p > 0,05 (p = 0,109 > 0,05). Conclusion: this study concludes that there is no significant relationship between the oral health knowledge and the severity of dental caries on the schoolchildren aged 12th to 15th.