Exposure to ultrafine particles poses a potential health risk to school children. While many studies have focused on measuring ultrafine particle (UFP) concentrations in environments where children are at risk of high exposure, few studies have examined the particle deposition in the respiratory tract of children. This study aims to examine the particle deposition in the respiratory tract of school children in different microenvironments. UFP size distribution data were collected in residential homes, school buses, school classrooms, and from school outdoor air in both rural and urban areas of South Texas. The size distribution data were input to the Multiple Path Particle Dosimetry model to calculate regional and total particle deposition fraction. A 24-hour-school-day exposure was simulated by adding the time children spend in each microenvironment. The maximum pulmonary deposition fraction occurs at a diameter ranging from 18 to 40 nm, depending on condition. Age mostly affected the pulmonary region and the total lung deposition with the highest deposition fraction observed for younger children. In addition, comparison of size-dependent regional deposition and particle concentration establishes an accurate depiction of children’s exposure and dose profiles. While children only spend 4% of the day in the home source environment, that environment may account for up to 77% of total daily dose intake. Higher deposition fraction occurs at smaller particle size. Younger children show increased levels of particle deposition than older children. Exposure period does not correlate to daily percentage of dose intake. This research can be used to assess children’s accumulative exposure to UFPs.