Understanding the role of transport and photochemistry is essential to alleviate regional ozone pollution. However, budget studies often report conflicting conclusions. Using the modeling results of WRF-CMAQ, we calculated the contributions of both processes to the variation of total ozone mass and mean ozone concentration (noted as ozone mass and concentration budget, respectively) within the atmospheric boundary layer (ABL) of the Pearl River Delta, China. Transport, especially the exchange between ABL and free troposphere, controls the ozone mass budget, whereas local photochemistry drives the rapid increase of ozone concentration in the daytime. Though transport has a limited effect on ozone concentration, its high contribution to the ozone mass budget determines that most ozone emanates from the outside regions. Consequently, the role of transport and photochemistry in ozone pollution may differ, depending on which of the two budgets is considered. Attention should be paid to budget type selections in future studies.