The Rank/Size rule (Zipf’s law) implies that the bigger the city, the more attractive the city. And, the difference of population growth in megacities all over the world implies that attraction of Asian megacities is greater than Western megacities. This research adds the “urban dream” factor into the push-and-pull theory for city formation, and because most of the “urban dream” comes from poverty, this research proposes that the intensity of urban dream is a decreasing function of human and economic development, thus the intensity of urban dream will be bigger in Asia, consequently the push-and-pull model with urban dream factor can explain the differences between Eastern and Western megacities.