In this note, we consider event-triggered extended state observer (ET-ESO) design for a continuous-time nonlinear system with uncertainty and disturbance. Since the convergence of the ET-ESO depends on the design of the triggering condition directly, the goal is to design a realizable triggering condition not relying on the state of the plant. For the extended state observer considered, an event-triggered transmission strategy is proposed such that the observation error is uniformly bounded. Furthermore, under mild conditions, the observation error can be guaranteed to be bounded at any time. It is shown that there is no Zeno behavior for the event-based transmission strategy. The obtained theoretical results are validated through numerical simulations.