A flexible organic electroluminescent device is disclosed which includes: a flexible substrate; a buffer layer entirely formed on the flexible substrate; a thin film transistor formed on the buffer layer and configured to include an active layer; a planarization film formed to cover the thin film transistor; an organic light emitting diode formed on the planarization film and configured to include a first electrode, an organic emission layer and a second electrode; and at least one silicon nitride layer formed above the active layer of the thin film transistor but under the planarization film and patterned into a plurality of island patterns.