In this retrospective study, children aged <20 years at the time of type 1 diabetes mellitus diagnosis, from March 2007 to March 2019, were included. The crude and adjusted incidence rate of type 1 diabetes mellitus is calculated as the number of cases per 100,000 person-years by the period. The cumulative, age- and sex-specific incidence rates were also calculated. Age-specific incidence rates were calculated for age and sex groups. A total of 1,954 (983 boys and 971 girls) cases of type 1 diabetes mellitus were identified. The mean age at diagnosis in all studied populations was 9.89 (standard deviation 4.76). There were no significant differences between the proportion of boys and girls in different years (P = 0.12) and different age groups (P = 0.19). The average annual percent change of incidence rate for the total population, for girls and boys, was 6.9%, 6.7% and 6.3% respectively. The type 1 diabetes mellitus incidence rate had a significant trend to be increased from 2007 to 2019 (P < 0.001, t = 3.6). Our findings showed that currently our region is considered a region with a high incidence rate of type 1 diabetes mellitus. Although we have had fluctuations in the incidence rate over the 12 years, the overall trend is increasing.