In the present study, the condensation of a newly developed low-GWP refrigerant R1234ze(Z) in a vertical plate-fin heat exchanger has been investigated experimentally and theoretically. In the experiment, the local condensation heat transfer characteristics of R1234ze(Z) flowing downward inside a finned vertical path made of SUS 316 were measured with a pressure transducer, some thermocouples and some heat flux sensors. The experimental results were compared with a semi-empirical correlation of mixed convection condensation heat transfer of pure refrigerant inside a vertical smooth tube, which were proposed by Dilao (1993). The experimental results were about two times higher than the semi-empirical correlation. In the theoretical analysis, the conjugate problem of the mixed convection condensation on a finned vertical surface and the heat conduction in a fin was solved numerically. Then, the numerical results on the heat transfer characteristics were compared with the experimental ones. The effects of fin shapes on the heat transfer characteristics were also examined by the present numerical analysis.