Gallium nitride (GaN) single crystal has the characteristics of high breakdown voltage, direct band gap, high saturated electron drift rate, good chemical stability, etc., and has been widely used in optoelectronic devices and high-power electronic devices. However, the heteroepitaxial of GaN produces a high dislocation density, which limits the performance of GaN devices. In this study, GaN single crystal was grown by ammonothermal method using HVPE-GaN as a seed crystal. The dislocation evolution from the seed crystal region to the lateral growth region of GaN single crystal was studied by scanning electron microscopy (SEM), optical microscopy and wet etching. The results show that the dislocation density of GaN single crystal in the lateral growth region is obviously lower than that in the seed crystal region, and the dislocation density decreases by 2 orders of magnitude when the lateral growth exceeds 25 μm.