Voltage island has become a very effective design style for power saving by assigning different supply voltages to different modules. However, the new design style also brings forward new challenges, especially to the designer of P/G networks. In this paper, we study the power delivery problem in voltage island designs, and propose to consider voltage drop during the floorplanning process to reduce design iterations. We have implemented a floorplanner supporting voltage islands with special considerations on power delivery integrity. Our analysis shows that it is unnecessary to consider the pitch of the P/G network in the floorplan stage. Using this strategy to guide our new floorplaner, we can get more robust low power design under almost the same run time. Experimental results have demonstrated the effectiveness of our approach.