We have achieved a thick AlGaN/GaN HEMT on Si substrate using GaN/AlN multilayer. The multilayer is effective in relaxing the stress in the upper GaN layer. The vertical and horizontal breakdown voltages increased with the increase of the epitaxial layer thickness. A breakdown field of 1.8×10⁶ V/cm was estimated from the vertical breakdown voltage. The horizontal breakdown voltage as high as 1813 V was obtained across 10 μm ohmic gap. We also reported on the influence of deep pits on breakdown of AlGaN/GaN HEMTs on Si. For devices with deep pits, the breakdown was greatly affected by large leakage through buffer and substrate. Cross-sectional transmission electron microscopy image revealed that deep pits originate from Si because of Ga etching Si substrate at thermal cleaning. The three terminal-off breakdown decreased rapidly as the increase of density of deep pits. Both thick and pit- free epitaxial layer are important for the fabrication of device with high- breakdown.