We observed the carbon clusters by TEM after capturing them on a grid membrane during the oxyacetylene flame diamond synthesis. The clusters of similar to1.5 nm captured on the amorphous membrane in the gas mixture of the acetyleneto-oxygen ratio of 1.04 were mostly amorphous with a few having a diamond lattice. The clusters larger than 5 nm captured at the gas ratio of 1.09 were mostly graphite with a minor fraction of diamond. (C) 2002 Elsevier Science B.V. All rights reserved.