Treeing in low density polyethylene (LDPE) filled with alumina nanocomposite as well as unfilled LDPE samples stressed with 50 Hz ac voltage has been studied. The tree inception voltage was monitored for various samples with different nano-filler loadings and it is seen that there is an increase in tree inception voltage with filler loading in LDPE. Treeing pattern and tree growth duration for unfilled and nano-filled LDPE samples have also been studied. Different tree growth patterns as well as a slower tree growth with increase in filler loading in LDPE nanocomposites were observed. The observed slow propagation of tree growth with filler loading is attributed to the changes in the polymer crystalline morphology induced by the presence of nano-particles and the greater ability of the nanoparticles to resist discharge growth. SEM studies carried out to determine the morphology of unfilled and nano-filled LDPE showed an increase in lamellae packing in LDPE nanocomposites and this increased lamellar density leads to a reduction in the tree propagation rate. Partial discharge activities were also monitored during the electrical tree growth in both the unfilled and the nano-filled LDPE samples and were found to be significantly different. PD magnitude and the number of PD pulses per cycle were found to be lower with electrical tree growth duration in LDPE nanocomposites as compared to unfilled LDPE. The same trend was seen with increased filler loading also.