The investigation of electromagnetic fields (EMF) in human health risk assessment include epidemiology, human and animals, cellular system, biochemistry and molecular biology, and biophysical mechanisms. The causal chain describes a series of sequential steps to lead to disease outcome for EMF to cause biological effect. To complete the first step, or say primary effect, the physical reaction between EMF with bio-molecules must come into being chemical reaction in organism, and then bring biological reaction. To research relationship of radical, energy and cytosolic calcium has significance to know primary effect of EMF. The effect of exposure to 48 h at 0.1mT, 0.5mT and 1.0mT power frequency EMF on rat-hippocampal neurons was investigated by evaluating the reactive oxygen species (ROS) and cytosolic Ca²⁺ concentration. The results show that the reactive oxygen species (ROS) and cytosolic Ca²⁺ concentration at 0.1mT, 0.5mT and 1.0 mT EMF have be all signification increaser (P < 0.01) than control (sham exposure), ROS at 0.1mT and 0.5mT and Ca²⁺ at 0.1mT have be no difference (P > 0.05) than Trolox + EMF, ROS at 1.0 mT and Ca²⁺ at 0.5mT and 1.0 mT have be signification increaser (P < 0.01) than Tolox + EMF. It is known that EMF may enhance ROS formation in neurons, and there is positive correlation between ROS level and cytosolic Ca²⁺ concentration.