Thin films were prepared for the first time from cobalt tetraphenylporphyrin (CoTPP) by plasma polymerization at different r.f. powers. They had cross-linked structures in which the porphyrin ring structure of CoTPP monomer was retained to different extents. Studies by use of a sandwich type device of Au/plasma polymerized CoTPP thin film/ITO glass revealed that plasma polymerized CoTPP thin films had semiconducting characteristics of p-type. The plasma polymerized thin film prepared at an r.f. power of 10 W exhibited better rectification characteristics than those prepared at higher r.f. powers.