We demonstrate the single-operation growth for diameter-tuned and site-defined nanowires by combining in-situ diameter-tuned technique with our self-assembly site-defined process. By using the self-catalyzed vapor-liquid-solid approach, the indium particle size and the nanowire diameter can be tuned by modulating the flow rate of the indium source material. This work realizes a new functionality of bridging the gap between the microscale and nanoscale features.