It is well known that space charge accumulation has a great influence on both the short-time and long-time breakdown strengths of plastic insulation systems operating under direct voltages. In this paper, dc treeing phenomena initiated from the tip of a needle electrode inserted into a polyethylene specimen have been investigated under various conions. The characteristics of four kinds of trees, dc tree, short-circuit tree, polarity reversal tree and impulse tree, were investigated as a function of the voltage-rising speed, period of dc prestress, time before the short-circuit or the application of a voltage of reversed polarity, impulse voltage, needle electrode material and temperature, etc. Also, thermally stimed current (TSC), thermoluminescence (TL) and photo-induced current techniques were applied to estimate the nature of the trapped carriers and carrier trapping centers. From the experimental results, it was concluded that a homo-charge is formed at high electric stresses and that the homo-charge induces an electric field distortion around the needle tip. In order to explain the length of the tree as a function of the various factors mentioned above, a model for the space charge formation is proposed, including the process such as charge injection, trapping and its diffusion.