Vapor pressures for new low-GWP refrigerants of R1233zd(E) (trans-1-chloro-3,3,3-trifluoropropene) and R1336mzz(Z) (cis-1,1,1,4,4,4-hexafluoro-2-butene) were measured by the isochoric method. Twelve vapor-pressure measurements of R1233zd(E) were obtained at temperatures between 300 and 410 K, whereas 17 vapor-pressure measurements of R1336mzz(Z) were obtained at temperatures between 290 and 410 K. Temperature was measured with a 25 Omega standard platinum resistance thermometer with an aid of ITS-90, and pressure was measured with a digital quartz pressure transducer. The experimental uncertainties in temperature and pressure are estimated to be within 5 mK and 1 kPa, respectively. The experimental data were compared with the previous experimental data and equations of state. New vapor-pressure correlations of R1233zd(E) and R1336mzz(Z) have been formulated on the basis of the data obtained.