In this study, we report on using a 50-MHz functional photoacoustic microscopy (PAM) to transcranially image the cross-section and hemoglobin oxygenation (SO2) changes of single mouse cortical vessels in response to left forepaw electrical stimulation. Three difference levels of the cortical vessels (i.e., with different-sized diameters of 350, 100 and 55 mu m) on activated regions were marked to measure their functional cross-section and SO2 changes as a function of time. Electrical stimulation of the mouse left forelimb was applied to evoke functional changes in vascular dynamics of the mouse somatosensory cortex. The applied current pulses were with a pulse frequency of 3 Hz, pulse duration of 0.2 ms, and pulse amplitude of 2 mA. The cerebrovascular cross-section changes, which indicate changes in cerebral blood volume (CBV), were probed by images acquired at 570 nm, a hemoglobin isosbestic point, while SO2 changes were monitored by the derivatives of 560-nm images normalized to 570-nm ones. The results show that vessel diameter and SO2 were significantly dilated and increased when compared with those of the controlled ones. In summary, the PAM shows its promise as a new imaging modality for transcranially functional quantification of single vessel diameter (i.e., CBV) and SO2 changes without any contrast agents applied during stimulation.