Despite the enthusiastic pursuing for large wind turbine blades to reduce the cost of wind power, wind energy industry has witnessed a number of catastrophic blade failure accidents in recent years. In order to provide more insights into the failure of large blades, this short communication presents preliminary investigation on a 52.3 m composite blade designed for multi-megawatt wind turbines. Static loads were applied to simulate extreme load conditions subjected by the blade. After blade failure, visual inspection was carried out and failure characteristics of the blade were examined. It was found that the blade exhibited multiple failure modes. Among various failure modes observed, delamination of unidirectional laminates in the spar cap was identified to be the plausible root cause of the catastrophic failure of the blade. This study emphasized that through-thickness stresses can significantly affect the failure of large composite blades and provided some suggestions to the current design practices. (C) 2014 Elsevier Ltd. All rights reserved.