There are limited information on caries incidence, especially from developing countries, the aim of the present study was to explore caries incidence in the first permanent molar teeth according to the CAST index in 7- to 8-year-old-children and its socio-demographic, oral health related and diet determinants. A multi-stage cluster random sample of 7-8 years old children was applied in Tehran, Iran. The oral examination using the CAST index and the Oral Hygiene Index-Simplified (OHI-S) performed by trained dentists in 2017 and 2019 calibrated with an expert (Kappa of 0.89 and 0.76, respectively). A 3-day food record was used to record sugary snacks consumption. Oral health related knowledge of the parents was assessed using a valid and reliable self-administered questionnaire. The data were analyzed using the SPSS software version 23.0 and descriptive and analytical statistics including the negative binomial regression was applied. Two hundred and ninety schoolchildren aged 7-8 years old were followed up for two years. All of them had complete data obtained via oral examination and questionnaires. The annual caries incidence rate was 0.16 and 53% (95% CI 47.4-58.9) of the children developed at least one new dental caries (enamel or dentine) during two years. Multi-variate analysis revealed that the children of mothers with high school education or diploma (IRR = 1.47, 95% CI 1.02-2.12; p = 0.04) and those with low socio-economic status (IRR = 1.86, 95% CI 1.27-2.73; p < 0.001) were more likely to develop caries. There was no significant association between gender, father's educational level, child birth order, housing area per person, OHI-S score, oral health knowledge of parents, and sugary snacks consumption per day and caries increment at an individual level. This 2-year longitudinal study on 7- to 8-year-old children showed that caries incidence according to the CAST index was associated with socio-economic status and mother education but not associated with having 2 or more sugary snack per day and oral hygiene status.