Control of final organ size is a fundamental and core process of development of all multicellular organisms, but the mechanisms that set the final size of determinate organs are largely unknown. In a recent study, we demonstrated that the Mediator complex subunit 25 (MED25/PFT1), which is involved in the transcriptional regulation of gene expression, controls the final size of determinate organs by restricting both cell proliferation and cell expansion. med25 mutants formed large organs with larger and slightly more cells, whereas plants overexpressing MED25 produced small organs due to a reduction in both cell number and cell size. Here, we show that a loss-of-function mutation in the Mediator complex subunit 8 (MED8) causes small flowers as a result of reduced cell expansion. Analysis of the med8 med25-2 double mutant reveals that MED8 acts independently of MED25 to regulate cell expansion and organ growth. Taken together, our findings show that MED8 and MED25 play an important role in regulating organ size. Further identification of upstream and downstream components of MED8 and MED25 will help understand how the Mediator complex is involved in organ size control in plants.