Seven carbon fibers were embedded into two different types of epoxy resin to prepare microcomposites, one epoxy being stiff and the other being flexible. These microcomposites were used for short-term and long-term strength tests. The long-term test results were conducted at three stress levels. The experimental results indicated that matrix properties play an important role in the mechanical behavior of composites. It was found that the stiff epoxy produces significantly stronger and creep rupture resistant composites. (C) 2004 Elsevier Ltd. All rights reserved.