Localization challenges researchers for its contradictive goals, i.e., how to tackle the problem of minimizing energy consumption as well as maintaining localization precision, which are two essential trade-offs in wireless sensor network systems. In this paper, we propose an Energy-Efficient Opportunistic Localization (EEOL) scheme to satisfy the requirement of positional accuracy and power consumption. We explore the idea of opportunistic wakeup probability to wake up an appropriate numbers of sensor nodes, while ensuring the high positional accuracy. Sensor nodes can be triggered by the opportunistic wakeup probability sent from the user. Through utilizing this method, the number of active sensors in the sensing range of the user is decreased, and the power consumption is significantly reduced. Theoretical analysis has been presented to evaluate the performance of EEOL. Simulation results show that EEOL confirms our theoretical analysis.