The activation energy and initial growth of electroless cobalt thin films on silicon wafer has been studied by transmission electron microscopy (TEM). Electroless cobalt was deposited on Si(001) substrates after preactivation treatment. The electroless cobalt deposited initially nucleated into particles with different sizes and shapes, which coalesced together sequentially, and finally became a continuous film. The incubation period of Co-P deposit decreased when plating temperature increased from 65 to 90degreesC. The plot of film thickness vs. plating time was used to determine the plating rates. The plating rates increased when plating temperature increased front 65 to 90degreesC. The activation energy for the initial deposition of electroless Co-P film was 15.67 kcal/mol, which was calculated according to the Arrhenius equation. (C) 2004 The Electrochemical Society.