© 2020 Elsevier B.V. Zn(Te1−xSex) quantum dots (QDs) were synthesized by the hot-injection method using zinc acetate as a zinc source. The composition, x, of the Zn(Te1−xSex) QDs was easily controlled in the range from 0 to 0.28 by changing the molar ratio between Te and Se precursors in the reaction solutions. The Zn(Te1−xSex) QDs with x = 0.24 ± 0.04 having various sizes (diameters from 3.8 to 6.0 nm) exhibited size-dependent photoluminescence (PL) emission with a peak wavelength in the visible region from 530 to 579 nm. The PL emission band involved not only the band-edge emission, but also defect-related emission. The defect responsible for the PL emission was attributed to the Te dangling bond on the surface of the QDs. The electronic transition for the surface state PL emission was concluded to be the transition from the 1Se quantum level to the acceptor level arising from the Te dangling bond.