The temperature dependence of pseudo-first-order rate constants (k_cyc) of the cyclization of hexanucleotides 5'-d(pGCGCG)rC and 5'-d(pGCCCG)rG was investigated in the presence of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDAC) and imidazole at 0 - 75 ℃. Although the association between the elongating oligomer and the activated monomer is necessary for the success of the formation of oligonucleotides on a polynucleotide template or a clay catalyst, the phosphodiester bond formation in the case of cyclization readily occurs without these catalysts. Thus, the cyclization is regarded as a simple model of the formation of phosphodiester bond. The rate constants (k_cyc) were compared with those of the cleavage of the ribose phosphodiester bond, in which the cyclization was more than 50 times faster than the cleavage at 75 ℃. On the basis of this fact, it is deduced that the prebiotic formations of RNA oligomers such as the template-directed reaction and the mineral catalyzed oligomerization would be possible at high temperatures if the association between elongating oligomer and activated monomer (or activated oligomer) is sufficiently strong.