This study applied trait–state–occasion (TSO) modelling to investigate the extent to which inter‐individual differences in personality were accounted for by time‐invariant and time‐varying factors during adolescence. The participants were 753 high school students, and only 661 students were followed for 4 years. The mean age in the first year was 15.23 years and 52.8% were females. The results showed that the full TSO model had the best representation of adolescent personality, and the means of the proportions of variance explained by trait factors for the Big Five ranged between 56% and 83%, suggesting that inter‐individual differences in personality are mainly accounted for by stable trait variance over the course of adolescence. Furthermore, the five dimensions of personality differed not only in the amounts of trait variance but also in the stability of time‐varying factors. These findings have significant implications for personality research.