To investigate the mechanism of C4F8 dissociation in parallel-plate-type plasma, we used several of the latest diagnostic tools and. made extensive measurements of electrons, radicals, and ions under conditions that greatly suppressed the effects of plasma-surface interaction. These measurements showed that the amount of light fluorocarbon radicals and ions increased with increasing electron density. The dissociation of C4F8 was analyzed by using rate equations, after confirming the stability and uniformity of the plasma. The total dissociation rate coefficient of C4F8 was 1 x 10(-8) cm(3)/s, and, CF2 radicals were mainly generated from products of C4F8 dissociation. F was mainly generated from CF2 by electron-impact dissociation and lost by pumping. We could estimate that the C2F4 density was roughly comparable to the densities of CF and CF3, and that the surface loss probability of C2F4, increased with increasing electron density. C2F4 might play an important role in the etching because of its rich polymerization characteristics.