Design and optimization of wideband nonlinear RF circuits are time-consuming and challenging design issue. Both electronic design automation (EDA) and numerical analysis tools can be used to design microwave circuits. However, they have different capabilities regarding calculation and optimization processes. In this study, EDA and numerical analysis tools are operated collaboratively and sequentially to create an automated co-simulation based iterative design and optimization methodology for wideband power amplifier design. Co-simulation based optimization provides uninterrupted and un-intervened design and optimization process. Consequently, the required effort and time to achieve a high-performance circuit are reduced dramatically. Keysight ADS is used nonlinear analysis of the circuits and MATLAB used to control process and for optimization. Initial circuit parameters were generated by simplified real frequency technique (SRFT). A Class-AB X-Band high power amplifier including a GaN HEMT was designed and optimized with the proposed methodology.