The effect of metal thickness on the quality (Q-) factor of the integrated spiral inductor is investigated in this paper. The inductors with metal thicknesses of 5 similar to 22.5 mum were fabricated on the standard silicon substrate of 1 similar to 30 Omega (.) cm in resistivity by using thick-metal surface micromachining technology. The fabricated inductors were measured at GHz ranges to extract their major parameters (Q-factor, inductance, and resistance). From the experimental analysis assisted by FEM simulation, we first reported that the metal thickness' effect on the Q-factor strongly depends on the innermost turn diameter of the spiral inductor, so that it is possible to improve Q-factors further by increasing the metal thickness beyond 10 mum.