Experimental data and mathematical model have been used in the present analysis to understand the condensation characteristics, especially the heat transfer degradation of binary zeotropic mixtures of R1234ze(E) and R32 inside a horizontal smooth tube of which inside diameter is 4.35 mm. The variations of the mass fractions of mixtures R1234ze(E)/R32 are 0.25/0.75, 0.55/0.45, 0.75/0.25 and the variations of mass fluxes of refrigerants are 50〜400kg m^<-2>S^<-1>. The temperatures and concentrations at the bulk and interface regions are obtained to understand the heat transfer degradation due to the mass transfer resistance of both the vapor-side and liquid-side. Considering the heat transfer degradation effect, the experimental local heat transfer coefficients are predicted by the obtained interface temperatures and correlation of pure refrigerant condensation.