Anatomical characteristics of vessels have a profound impact on the hydraulic conductivity of the xylem. However, pore shape, the cross-section shape of a vessel, has been ignored in previous hydraulic architecture studies. In this study, we examined whether pore shape tended to be circular, and whether this variation tendency might be affected by water flow path-length and cambial age. The circularity pores in Betula platyphylla Roth and Quercus mongolica (only earlywood) were analyzed from the pith to the bark along the water flow path. It was found that although there were very few pores with absolutely circular shape, solitary pores did tend to be regularly circular. The pore shape of sequentially formed vessels in the xylem was influenced by cambial age and flow path-length. Pore shape tended to be more circular as flow path-length increased or cambial age decreased. Moreover, circular pore shape is an important morphogenetic mechanism for hydraulic architecture development, which can also expand the understanding of structure-function relationships for the xylem.