To help wind turbine reliability analysis in a design stage, aeroelastic simulators have been developed to generate stochastic load responses imposed on a wind turbine structure, given pre-specified turbulent wind conditions. In particular, system designers can use the simulators to estimate the extreme load responses during a turbine's design life. However, it has been shown that using the crude Monte Carlo sampling methods to simulate the extreme load associated with a small load exceeding probability is computationally prohibitive, and the estimation results are highly uncertain. Importance sampling methods can overcome the limitations of the crude Monte Carlo sampling methods. We develop adaptive algorithms to iteratively refine the importance sampling density to efficiently estimate the extreme load. Nomenclature P T Target probability of load exceedance l T Extreme load X Input vector Y Output variable p X Original input density of X q X Importance sampling density θ Parameter in the importance sampling density