In the preparation of Nicalon-type Si-C-O fibers using a normal-pressure synthesized polycarbosilane precursor, a network of microflaws were detected on the cross-section of ceramic fibers obtained. Cross-section morphologies of the interior microflaws were investigated via SEM technique after certain temperature pyrolysis and their initiation and propagation behaviors were studied accordingly. The results showed that no microflaws came into being at the temperature up to 600degreesC due to the organic nature of the material. As a result of high weight loss and composition change, however, microflaws initiated over 600degreesC, formed at similar to700degreesC and propagated along the "core-edge" direction from 700 - 900 degreesC. Up to 1100degreesC, a microflaw network was formed because new microflaws generated across the oriented ones and interconnected them to form a network. (C) 2004 Kluwer Academic Publishers.