To estimate the radial variation of axial conductance in the SPAC, diurnal trends in the sap flow velocity at the different depths in stem xylem and the xylem pressure potential were measured for three coniferous tree species and three broad-leaved tree species. Except for ring-porous wood species, the sap flow velocity increased with proximity to the cambial zone except in close proximity for coniferous and broad-leaved tree species. In the ring-porous wood species, the sap flow velocity in the earlywood vessels was larger than that in the latewood vessels. The sap flow velocity in the earlywood vessels of the current year’s growth ring was larger than in those of 1-year-old growth ring. The resistance to water flow at various depths in stem xylem of the SPAC could be estimated from a relationship between xylem pressure potential and heat pulse velocity. The resistance in the latewood vessels of Q. acutissima was about 4 to 7 times as large as that in the earlywood vessels. It can be estimated that the earlywood vessels will play an important role in water conduction of ring-porous wood species.