Herein, we have directly investigated a 2 delta L as the diffusion length in a‐IGZO TFT using a scanning capacitance microscopy (SCM) technique at the cross section of TFTs instead of indirect normal transmission line method (TLM). Moreover, we revealed that the difference of 2 delta L leads the variation of total parastic capacitance in devices, which play a key role in the properites of EL currents in case of display devices.