For understanding the possibility of mixtures R1234ze(E)/R32 as low-GWP alternatives for residential or commercial heat pump systems, in this study we have experimentally evaluated the cycle performance of a water-heat-source vapor compression heat pump cycle using three refrigerants: a zeotropic mixture R1234ze(E)/R32 (20/80 mass%), a near-azeotropic mixture R410A, and a pure refrigerant R1234ze(E). The COP and capacity of the heat pump cycle were measured under cooling and heating modes. In cooling mode, the cooling load ranged from 1.4 kW to 2.4 kW, and in heating mode the heating load ranged from 1.6 kW to 2.8 kW. The two kinds of heat exchangers were tested in the experiment to clarify the effect of temperature glide of zeotropic mixture in phase change processes on the cycle performance. When the heat exchanger size is large enough, the temperature glide of zeotropic mixture is effective to reduce irreversible loss in heat exchanger. As a result, at some operating points, COP of R1234ze(E)/R32 (20/80 mass%) exceeds that of R410A and R1234ze(E). This supports the possibility of R1234ze(E)/R32 mixtures as the alternative of R410A.