This letter reports the nanoscale spatial phase modulation of Ga As growth in V-grooved trenches fabricated on a Si(001) substrate by metal–organic vapor-phase epitaxy. Two hexagonal Ga As regions with high density of stacking faults parallel to Si {111} surfaces are observed. A strain-relieved and defect-free cubic phase Ga As was achieved above these highly defective regions. High-resolution transmission electron microscopy and fast Fourier transforms analysis were performed to characterize these regions of Ga As/Si interface. We also discussed the strain relaxation mechanism and phase structure modulation of Ga As selectively grown on this artificially manipulated surface. This letter reports the nanoscale spatial phase modulation of GaAs growth in V-grooved trenches fabricated on a Si (001) substrate by metal-organic vapor-phase epitaxy. Two hexagonal GaAs regions with high density of stacking faults parallel to Si {111 A strain-relieved and defect-free cubic phase Ga As was achieved above these highly defective regions. High-resolution transmission electron microscopy and fast Fourier transforms analysis were performed to characterize these regions of Ga As / Si interface. We also discussed the strain relaxation mechanism and phase structure modulation of Ga As selectively grown on this artificially manipulated surface.