Background and purpose: Dental health education for mothers is very important in maintaining the dental health of preschool children. During the Covid-19 pandemic, there were limitations in the direct education process, so it is very important to conduct online dental health education. Purpose: This study purpose to compare the effectiveness of dental health education with audiovisual media, modules, and posters given online through Whatsapp media in improving the behavior (knowledge, attitudes and actions) of mothers in preventing dental caries in preschool children. Methods: Research subjects were determined by consecutive sampling according to those who met the inclusion and exclusion criteria. Total 73 subject of this study divided into three group intervention, 21 subjects in poster group, 23 subjects in modules group and 29 subjects in audiovisual group. The intervention was carried out by providing posters, modules, and videos of children's dental health education through online discussions in the Whatsapp group. Questionnaires were given before the intervention (pre-test), two weeks (post-test 1) and four weeks after the intervention (post-test 2). Results: Online dental health education with posters, modules and audiovisuals is effective in improving mother’s behavior in preventing dental caries in preschool children. Audiovisual and module method are more effective than poster to improve mother’s behavior in preventing dental caries in preschool children (p<0,001). Conclusion: Dental health education online with audiovisual media has the best effectiveness in improving maternal behavior in preventing dental caries in preschool children.