We report the low-frequency noise (LFN) behavior of amorphous indium–gallium–zinc oxide thin-film transistors with an inverse staggered structure and an SiO2 gate insulator. The normalized noise power spectral density depended on channel length, L, with the form 1/L
2, and on the gate bias voltage, V
G, and threshold voltage, V
TH, with the form 1/(V
G − V
TH)β where 1.5 < β < 2.1. In addition, the scattering constant α was less than 105 Ω. These results suggest that the contact resistance has a significant role in the LFN behavior and the charge-carrier density fluctuation is the dominant origin of LFN.