Methane, n-hexane, benzene, and cyclopentadiene were decompd. at a relatively mild temp. (773 K) over a Ni catalyst supported on either vapor-grown carbon fibers or graphitized carbon fibers. Transmission electron microscopy showed that the morphol. of the fibers changed depending on hydrocarbon and particle size. Decompn. of methane and n-hexane produced fishbone-type fibers. The fibers from n-hexane sometimes showed intermittent hollow structures but the diams. of the fibers were widely distributed. Decompn. of benzene and cyclopentadiene mainly produced winding type carbon nanotubes of relatively uniform diams. (10-20 nm). Bidirectional fishbone-type fibers (fibers growing outward from a central catalyst particle) were also obsd. as a byproduct. Small Ni particles (10-20 nm) with stretched tails were present on the tips of tubular fibers, some of which frequently changed growth direction. The varying tubular morphologies can be ascribed to liq.-like Ni particles resulting from the f.p. depression due to a fast dissoln. of carbons during decompn. of benzene or cyclopentadiene. The formation of bidirectional fibers was also obsd. in the decompn. of n-hexane. Relatively large well-faceted Ni particles (diam. 50-110 nm) grew bidirectional fibers. [on SciFinder(R)]