Knowledge is the result of knowing and this is after the person does the sensing of a particular object. Knowledge of dental caries is one way to create optimal dental health, by knowing the cause of caries, can prevent the occurrence of caries. Caries is a demineralization process caused by an interaction between microorganisms, saliva, food, time and teeth. The purpose of this study to determine the effect of knowledge on dental caries in patients at Puskesmas Leyangan district Semarang.The design of this study using descriptive method, with crosssectional approach, this study population is dental caries patients who come to Leyangan Puskesmas, samples taken as many as 34 people. The measuring instrument used in this study is a questionnaire to measure knowledge and examination sheet to determine the caries status. Research data are presented in frequency distribution and cross tabulation.The results showed that knowledge of dominant caries had moderate category with low caries category. The conclusion of this study is that knowledge is not related to the dental caries condition of a person. It is hoped that puskesmas will improve cooperation with dental officers to play an active role in society. For the community more increase knowledge about dental and mouth health.