Newly developed multi-layer inductors on GaAs three-dimensional MMICs are presented. We tested single-, double-, triple-, and quadruple-layer stacked-type inductors in what may be the first report on inductors on a GaAs MMIC with three or more layers. These proposed multilayer inductors can produce higher inductance in the same area as conventional 2D-MMICs. The performance of the single- and multilayer inductors was measured on wafer and calculated by electromagnetic field analysis using the finite element method. Though they are the same size, the multilayer inductors produce 2-11 times higher inductance than the single-layer inductors. In addition, the degradation of the multilayer inductor performances, such as resistance, Q-factor and self-resonant frequency, was not seen compared with the single-layer inductors. Multilayer inductors make it possible to shrink circuit size from many hundreds of MHz to the millimetre wave.