Two-dimensional (2D) ReS2 flakes are synthesized by the chemical vapor deposition under atmospheric pressure using Re–Te binary eutectic and sulfur as precursors. The morphologies, crystal structure, phonon vibration modes, chemical states and optical property of the 2D ReS2 flakes are characterized using optical microscopy, scanning electron microscope, transmission electron microscopy, the Raman spectroscopy, X-ray photoelectron spectroscopy and transmittance spectra. The lateral size of 2D ReS2 flakes can be tuned via changing the synthesis parameters. The monolayer 2D ReS2 flakes is single crystal structure. Its maximum lateral size is 30 μm, and optical band gap is 1.42 eV. The absorption coefficient is as large as 105 cm−1, transmittance of 92–98% and refractive index of 1.5–1.3 in the wavelength range from 400 to 800 nm. The effect of synthesis parameters on the morphologies and growth mechanism of 2D ReS2 flakes are investigated in detail. This is helpful for further application of ReS2 in electronic and optoelectronic devices.