The puzzle of why hypothesis such as dominance or over-dominance cannot explain the heterosis mechanism properly was discussed. It is considered that global heterosis, such as heterosis of yield, does not necessarily be the result of local heterosis which could be explained fully by these hypothesis. It is proposed that, among the local traits, there is a multiplicative relationship by which the heterosis of local traits could be organized to form global heterosis .And thus, multiplicative relationship and multiplicative effect are considered to be the important basis of global heterosis. Some suggestions concerning breeding practice of hybrid rice were further discussed.