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In this work, structure, optical, and thermoelectric properties of layered ZrS_2−xSe_x single crystals with selenium composition of x = 0, 1, and 2 were examined. Single crystals of zirconium dichalcogenides layer compounds were grown by chemical vapor transport method using I₂ as the transport agent. X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) results indicated that ZrS_2−xSe_x (x = 0, 1, and 2) were crystalized in hexagonal CdI₂ structure with one-layer trigonal (1T) stacking type. X-ray photoelectron and energy dispersive X-ray measurements revealed oxidation sensitive behavior of the chalcogenides series. Transmittance and optical absorption showed an indirect optical gap of about 1.78 eV, 1.32 eV, and 1.12 eV for the ZrS_2−xSe_x with x = 0, 1, and 2, respectively. From the result of thermoelectric experiment, ZrSe₂ owns the highest figure-of merit (ZT) of ~0.085 among the surface-oxidized ZrS_2−xSe_x series layer crystals at 300 K. The ZT values of the ZrS_2−xSe_x (x = 0, 1, and 2) series also reveal increase with the increase of Se content owing to the increase of carrier concentration and mobility in the highly Se-incorporated zirconium dichalcogenides with surface states.