The electronic and optoelectronic properties of MoS2 and WS2 devices and MoS2/ WSe2 heterojunctions have been studied in this thesis. In transition metal dichalcogenides (TMDC) field effect transistors using ionic liquid as the dielectric layer ambipolar transport behaviors are observed, which allows us directly extract the energy gap of the thin films. In addition, the Schottky barrier height can be tuned by controlling the doping level of TMDCs and almost eliminated when a positive gate voltage is applied. The further study of the magneto transport property lead us to observe weak antilocalization effect. Moreover, MoS2/WSe2 p-n junction fabricated by polymer-free dry transfer method shows clearly a diode like rectification behavior and can be well tuned by back-gate voltage. Upon optical illumination, both charge transfer process and recombination process occur at the junction region. By further comparing the photocurrent result and the (photoluminescence) PL under different back-gate voltages, interlayer exciton recombination process and the interlayer charge transfer process are found to compete against each other. Our transport measurements show that both 2D devices or heterostructure have potential for future application.