This paper reports the realization, radio frequency characterization and modeling of flexible spiral inductors on low cost, flexible plastic substrates. The spiral inductor on plastic substrate exhibits better performance compared to the one on conventional silicon substrates. The peak Q value of the 6.2 nH flexible spiral inductor is 14.2 and the selfresonant frequency is up to 9.2 GHz. An equivalent circuit model of flexible spiral inductors on plastic is presented. And the equivalent circuit model achieves good agreement with the experimental results. The optimization methods of improving the performance of flexible spiral inductors are presented by the simulation of model parameters. The study provides guidelines for properly designing and using spiral inductors in flexible circuits and systems.