Effect of strain on space charge (SC) layer in nanowires (NWs) has been examined by in situ off-axis electron holography,where GaN NWs attach to an Au electrode inside a transmission electron microscope (TEM).Based on the phase image reconstructed from the complex hologram,the width of SC layer in a strained GaN NW is significantly reduced to about 60 nm,comparing to the 85 nm of the unstrained NW.About 29％ reduction of the SC layer in the strained GaN NW resulted from significant decrease of electrons flowed from the GaN into Au.First principle calculations show that the strain reduced bandgap of GaN,narrowing the difference between GaN NW and Au electrode in Fermi level.