Elemental direct combination to synthesize ZnSe and CdSe only occurs at very high temperature. However, in this paper, a simplest hydrothermal elemental-direct-reaction method to produce nanocrystalline ZnSe and CdSe is established, only using water as reaction medium. It greatly decreases the temperature for the direct reaction of Zn, Cd with Se and, will be helpful to the synthesis of other nanocrystalline semiconductors.