A polyacetylene with excellent stretchability can be obtained by a process which comprises polymerizing acetylene with a Ziegler-Natta catalyst consisting of a tetraalkyl titanate and a trialkylaluminum, wherein in the Ziegler-Natta catalyst at least one of the tetraalkyl titanate and the trialkylaluminum has a long chain alkyl group and the molar ratio of al to Ti is 1.5-10.
