SPEEK has been widely considered as a promising alternative to Nafion (R) membrane for PEMFC. However, a conflict between high degree of sulfonation (DS) and poor mechanical strength needs to be resolved prior to wide application. In this work, such an effort was made by double strategies: thermal crosslinking and adding nanodiamond into high DS SPEEK matrix. The obtained sample was characterized by XRD, high resolution TGA and dynamic mechanical analysis (DMA). Small part of -SO3H groups within SPEEK matrix participated in the crosslinking process by forming -SO2- bridge bond, while the rest large part of -SO3H groups still contributed to the proton transport. Mechanical and thermal stability of SPEEK membrane were modified by crosslinking-induced three-dimensional (3D) networks and interactions of SPEEK with nanodiamond. In addition, the water uptakes of composite membranes were also slightly improved possibly due to the capillary condensation in nanodiamond particles. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.