We report an optimized multi-step chemical vapor deposition process for growing MoS2 thin films. This process enables large-area processing, film patterning simply by using shadow masks, and precise control of the final thickness by changing the initial thickness of the first step of MoO3 film deposition. The structural characterization of the MoS2 films is performed with transmission electron microscopy and X-ray diffraction as a function of film thickness. MoS2 film with a thickness of 3 nm possesses a highly crystalline structure with a spacing of 0.62 nm. The crystallinity and orientation of the films are degraded with increased film thickness. Careful analysis by time-of-flight secondary ion mass spectroscopy reveals that a film with a thickness of 9 nm is not completely sulfurized, and unreacted MoO3 is left at the bottom of the film. These fundamental analyses coincide with the thickness dependence of thin-film transistor (TFT) performance. A TFT with the optimal film thickness of 3 nm achieves high performance, namely a carrier mobility of 0.57 cm(2) V-1 s(-1) and an on/off ratio of similar to 10(2).