In this study, the defect structure of periodic Si delta-doping (delta-doping) GaN films grown by low-pressure metalorganic chemical vapor deposition has been investigated by high-resolution X-ray diffraction. Rocking curves of five planes were investigated: (0 0 0 2), (10 (1) over bar 3), (10 (1) over bar 2), (10 (1) over bar 1) and (2 0 11), respectively. Pseudo-Voigt function was used to simulate the rocking-curve of every plane. The effects of Si delta-doping on the different types of dislocations were discussed. It was demonstrated that Si delta-doping significantly reduces the threading dislocations with a pure edge character, and induces no changes in the threading dislocations with a screw component. The results are consistent with AFM results. (c) 2005 Elsevier B.V. All rights reserved.