Electrical tree growth in epoxy resin under square waves superimposed with DC voltages is studied in this paper. Using the needle-plane configuration, a ±15 kV DC voltage superimposed on a 15 kV pk 50 Hz square wave was applied to grow electrical trees. A standard 50 Hz sinusoidal source was also used for comparison. A CCD camera imaged the electrical tree growth during testing and associated partial discharges were recorded. It was observed that the tree shape under 15 kV square-wave voltages combined with positive or negative 15 kV DC voltages are branch-like, whereas bush-branch trees grow under 15 kV bipolar square-wave voltages. Positive unipolar square-wave voltages resulted in faster growth than negative unipolar square waves. The different tree structures were also associated with distinct characteristic PD activity.