Monolayer molybdenum disulfide (MoS2) with large area and high quality have been grown using Chemical Vapor Deposition (CVD) method. The optical properties of MoS2 was characterized systematically and high performance n-type field-effect transistors were fabricated. The effect of applied electrical field on photoluminescence (PL) spectrum of monolayer MoS2 in the devices was also studied. The results showed that the strongest emission peak of the PL spectrum of monolayer MoS2 consist of two peaks, namely A- peak (Charged Exciton) and A peak (Intrinsic Exciton) at room temperature. The energy difference between the two peaks is about 35 meV. The main PL peak shows obvious redshift and intensity change via tuning the back-gate voltage from negative to positive. We also found that the intensity of these two peaks show opposite dependence on the back-gate voltage with spectrum analysis. These results were analyzed and we concluded that the carrier concentration of monolayer MoS2 can be effectively modulated by applied electrical field, which can further affect the intensity and shape of PL spectrum. These results provide significant basis for the research on the physical mechanism of the optical properties of two-dimensional material. Besides, the large-scale preparation of such devices also make it possible to apply these two dimension materials to optoelectronics devices and systems.