In recent years, interfacial doping with other atoms, molecules, and nanoparticles in molybdenum disulfide (MoS2) has been proven as a new route to explore the potential application of 2D materials in microelectronical devices. In this paper, we utilized a one-step chemical vapor deposition approach to synthesize monolayer MoS2(1-x)Se2x nanosheets in atmospheric pressure using MoO3, S, and Se powders as precursors. AFM and visible-light microscopy showed that the as-grown nanosheets were single layers, their surface was atomic flat, and the maximum grain size was over 100 mu m. XPS characterization demonstrated that the concentration of selenium in MoS2(1-x)Se2x nanosheets was affected by the amount of selenium powder in the doping process. The back-gate FETs were fabricated to investigate the electrical properties of monolayer MoS2(1-x)Se2x nanosheets with different Se contents. The field effect properties of MoS2(1-x)Se2x (x = 0.65) transistors indicated that a moderate mobility was achieved, and ohmic contact was obtained at the interface of the MoS2(1-x)Se2x channel and metal electrodes. Characterization using high-resolution transmission electron microscopy showed that the microstructure of as-grown MoS2(1-x)Se2x (x = 0.65) had a regular hexagonal lattice structure, which revealed that it was a single-crystalline two-dimensional material.