This paper proposes a novel environmental contour line method based on measured ocean wave data for predicting the long-term extreme wave conditions. For implementing the proposed environmental contour line method, a bivariate kernel density estimation approach is used along with over-smoothed bandwidth selection. The environmental contours obtained using the proposed method have been compared with those obtained using the Gaussian copula (Nataf transformation) method and another parameterized model, and the effectiveness and superiority of the proposed method have been clearly substantiated. The research results in this paper demonstrate that the proposed method can be utilized as an effective tool for predicting the long-term extreme wave conditions. (c) 2020 Elsevier Ltd. All rights reserved.