In order to clarify the mechanisms of partial discharge (PD) and tree degradation, PD characteristics of an artificially-simulated tree channel in low-density polyethylene (LDPE) were investigated. During the application of AC voltage, new tree channels were formed at the tip of the artificial channel. PD characteristics of an artificial channel with 2 mm in length and 30 µm in diameter remarkably changed after the inception of very small trees. After the tree inception, the number of PD pulses increased and light emission due to PD extended to a whole channel and new tree channel. A few minutes after the tree inception, the number of negative PD pulses decreased and its magnitude became large. These results suggest the change in PD mechanism by tree inception and tree propagation. From these results, the mechanisms of PD in a tree channel and tree propagation were discussed. These results are applicable to insulation diagnosis.