Dental caries is considered a major health problem among schoolchildren in Lao People's Democratic Republic (Lao PDR). According to Health Belief Model (HBM)-based research, children's oral health behavior can be determined by their guardians' beliefs. This study aimed to describe children's oral health behavior and its association with childhood dental caries, as well as to assess associations between children's tooth-brushing behavior and guardians' beliefs in an urban area of Lao PDR, using HBM. Data were collected from ten primary schools in the Sisattanak district, the Vientiane capital, between 2013 and 2014. Ten dentists with the help of dental hygienists and schoolteachers conducted dental health check-ups at the schools that diagnosed dental caries based on visual inspection. They also conducted a questionnaire-based survey with the schoolchildren's guardians to collect data including socio-economic and demographic information, their children's oral health behavior, and guardians' beliefs derived from HBM, including perceived susceptibility to and perceived severity of child dental caries, perceived benefit of and perceived barrier to child's tooth brushing, and self-efficacy in making their children brush their teeth twice daily. A mixed-effects logistic regression model assessed the association between dental caries and children's oral health behavior and between children's tooth-brushing behavior and guardians' beliefs. Data from 1161 of 1304 (89.0%) children registered at the schools were used. The prevalence of dental caries was 82%. Children who brushed their teeth ≥ twice/day were significantly less likely to have dental caries than those brushing once or seldom (OR: 0.64, 95% CI: 0.45 to 0.91). The number of children who brushed twice daily also significantly increased with the increased level of guardians' self-efficacy (OR: 2.14, 95% CI: 1.91 to 2.41). In conclusion, childhood dental caries was associated with daily tooth brushing. Children's tooth-brushing behavior was associated with guardians' self-efficacy in making their children brush twice daily.