The aim of this thesis is to introduce the mechanism ofstreamer process in dielectric liquids. The appearance of bushlike streamer shape firstly and then a tree structure later hasbeen given an interpretation and meaning. Also, the change ofstreamer velocity from tens of meter per second to severalkilometer per second, starting from bush like shape, treestructure, till complete breakdown has been introduced. ThesepreVIOUS stages have been studied on an energy balanceconcepts, from low applied voltage to high voltage values. Thelength of streamer also is studied and a model to estimate it hasbeen derived.When a high voltage is applied to a dielectric liquid inpoint plane configuration, a dark tiny branches spread out inmany direction outward from electrode tip toward planeelectrode. These tiny branches called bush like streamer. Thebreakdown criterion in dielectric liquid, in this thesis, is thecondition when the liquid zone around high voltage electrodetip (about five times of the bush branch radius) has beenevacuated and replaced by air. The number of branches whichshould fill this zone and empty it from liquid is 25 brancheswith 8 urn radius. This number of branches is considered asbreakdown index for dielectric liquids. According to energybalance concept; one branch channel in bush like streamer tobe initiated (41lm radius), the pressure inside it should begreater than the surrounding pressure. The required criticalelectrical field which achieves this condition is 2.18 MV/cm.when the applied voltage increases again, the tree streamerstructure appear as a tube emanated from bush like zone towardthe opposite electrode. Under the energy balanc~ concept; thecritical electric field to initiate a filamentary streamer is 21.5MY/cm for most hydrocarbon liquids.