The OpenMP specification introduces thread team for hierarchical parallelism. A thread team is a team of synchronizable threads, and the number of threads in a thread team is called thread team size. OpenMP allows static adjustment of the thread team size, where the team size must be specified before executing an application and has to stay constant until the end of the execution. Conventionally, the thread team size is decided by evenly dividing the number of running threads, and assigning them to each thread team. However, the conventional approach will face load imbalance across thread teams when it is applied to irregular applications. Therefore, assigning the same team size for every thread team and keeping the size constant during the whole lifetime of the thread teams could be ineffective. In this paper, we discuss the effects of a dynamic thread team size adjustment method for OpenMP. After dynamic thread team size adjustment, the number of threads in each team can be different. The evaluation results show that dynamically adjusting the number of threads in each team achieves a higher performance than using the same number of threads in each team, even if the runtime overhead is taken into account.