The time effect of ultrasonication was investigated for dispersing carbon nanofibers (CNFs) into a polycarbonate (PC) matrix on the mechanical properties of thus-produced composites. The effects of CNF surface modification by plasma treatment and the CNF concentration in composites on their mechanical properties were also explored. The plasma coating was characterized by HRTEM and FT-IR. Furthermore, the plasma polymerization (10 w) treatment on the CNF enhanced the CNF dispersion in the polymer matrix. The mechanical properties of the CNF-PC composites varied with the dispersion time, at first increasing to a maximum value and then dropping down. After a long ultrasonic treatment (24 h), the properties increased again. At a high concentration, the CNF-PC suspension became difficult to disperse. Additionally, the possible mechanisms for these behaviors are simply proposed. (c) 2006 Wiley Periodicals, Inc.