Ab initio calculations of the potential energy surface for the Cl+O-3 reaction have been performed using the MP2, QCISD(T), CCSD(T), G2, G2M, CASPT2, and MRCI methods with various basis sets. The results show that the reaction pathway can be divided in two parts. The reaction starts on the nonplanar pathway when the Cl atom attacks a terminal oxygen of ozone via TS1, producing a virtual intermediate, a nonplanar chlorine trioxide B. B isomerizes to another virtual intermediate, planar C, which immediately dissociates to ClO+O-2 in the coplanar manner. The ClOOO intermediates B and C disappear at the QCISD level of theory. The calculations confirm the direct reaction mechanism for Cl+O-3 but the existence of a very flat plateau on the potential energy surface in the region of B, TS2, C, and TS3 can have some effect on the reaction dynamics. TS1 is the critical transition state determining the rate of the Cl+O-3 reaction. High level calculations, such as QCISD(T), CCSD(T), MRCI, and CASPT2 with the basis sets from moderate to very large, at the QCISD and CASSCF optimized geometry of TS1, consistently predict the barrier to be about 4-5 kcal/mol, much higher than the experimental value (below 1 kcal/mol). New experimental measurements as well as even higher level theoretical calculations are encouraged in order to resolve this discrepancy.