A color-tunable light-emitting device using composi-tionally graded monolayer transition metal dichalco-genide alloy was fabricated. The monolayer WS2/WSe2 alloy was grown by chemical vapor deposition and showed bandgap variation from 2.1 eV to 1.7 eV, from WS2-rich side to WSe2-rich side. By using electro-lyte-based light-emitting device structure, the recombi-nation zone of the device could be tuned laterally. With the graded composition inside the channel region, the light-emitting device exhibited continuous and reversi-ble color tunability when emitting light at different voltages. Our results provide a new approach for the exploration of broadband optoelectronics based on monolayer semiconductors.