Recently the greenhouse and global warming effects are becoming more and more severe, that partially explains why rainfall rates are keep breaking record continuously in all parts of the world. Currently helicopter has widespread utilization by air rescue unit in Taiwan during disaster situation under extreme weather situations. For this reason thus the understanding and improvement of helicopters under heavy rain condition is the main theme of this research. With our understanding of heavy rain physics, CFD tool has first being developed and successfully tested on the standard helicopter blade. Once the heavy rain droplets are added, it is observed that rainfall indeed will induce some detrimental effects on the main rotor blade’s performance. Therefore, it is highly recommended that every helicopter pilot should be aware of this information gained in this work and has extensive training in some heavy rain situations.