Provided are a flexible thin film transistor substrate and a flexible organic light emitting display device. The flexible thin film transistor substrate includes: a flexible substrate including at least one thin film transistor (TFT) area and having flexibility, an active layer disposed in the TFT area on the flexible substrate, a gate insulation layer disposed on the active layer, a gate electrode overlapping with the active layer on the gate insulation layer, an insulating interlayer disposed on the gate electrode, and a source electrode and a drain electrode disposed on the insulating interlayer and connected with the active layer, respectively. The gate insulation layer or the insulating interlayer includes at least one hole pattern configured to reduce bending stress.