Grounded DC tree experiments and space charge measurement based on the pulsed electro-acoustic (PEA) method are performed on XLPE and 0.5% by weight MgO/XLPE nanocomposite. The tree experiment shows that MgO/XLPE nanocomposite exhibits higher 50% tree inception voltage under positive voltages. The space charge measurement shows that more homocharge is injected in XLPE, which indicates that nanoparticles impede the homocharge injection or heterocharge extration at needle tip, thus impedes the generation of electrical tree. And MgO/XLPE nanocomposite shows smaller average tree length and width when under positive voltages, which may be attributed to the carries generated from high stress region colliding with nanoparticles and slowing the tree propagation. Moreover, the average tree length-width ratio in MgO/XLPE nanocomposite decreases faster with the positive voltage increasing. This can be explained by the fact that electrical tree is hard to penetrate nanoparticles, but can only develop around their surfaces, which results in the change of tree channel direction and emergence of many thin lateral branches.