An obvious channel direction dependency of the low field hole mobility on (100) oriented silicon surface is experimentally obtained for p-channel metal-oxide-silicon field-effect-transistor (MOSFET) fabricated on atomically flattened silicon wafer. The low electric field hole mobility measured at a low temperature takes the maximal at  directions and the minimal at  directions, respectively. The obtained channel direction dependency agrees very well with that of the heavy hole effective mass. The correlations between the magnitude of channel direction dependency of the hole mobility and some physical parameters such as channel length, temperature, and lateral electric field are evaluated. As a result, a universal relationship was found between the mobility increase from  to  direction and the channel length over the average relaxation time constant of carrier scattering.