In this work Ga adatom incorporation diffusion length was quantified in two different directions, < 1 1 0 > and <1<(1)over bar>0>, during MBE growth of a GaP layer. Thickness distribution was measured for the GaP layer grown on striped patterned GaP substrate and the data was analyzed based on a one-dimensional diffusion growth model between two different adjacent facets. It was quantitatively revealed that the diffusion length of the Ga adatoms increased with growth temperature and an anisotropy in the diffusion length was observed along <110> and <1<(1)over bar>0> directions. The influence in the morphology due to the diffusion length anisotropy was discussed with the RHEED patterns.