Molybdenum oxide (MoOx) thin films were prepared by pulse DC sputtering using a molybdenum target and their optical and electrochromic properties were investigated. These MoOx thin films were reactively sputter-deposited at various oxygen flow rates. The effects of the oxygen flow rate on the growth and microstructure of the molybdenum oxide films were examined using X-ray diffractometry (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The optical density and modulation of the MoOx films were analyzed using a UV/VIS spectrophotometer (UV–visible). Electrochromic tests were conducted using an electrochemical analyzer. Experimental results reveal that the electrochromic properties depended strongly on the crystalline phase, thickness, chemical composition and microstructure, which were controlled by varying the oxygen flow rate. •MoOx thin films were deposited by bipolar pulsed magnetron sputtering of pure Mo targets.•Increasing oxygen flow rate resulted in the crystal structure of MoO3 which favors the electrochromic properties.•The maximum optical modulation for 25-sccm oxygen partial pressure coating are 19.2% at 400 nm, 36.2% at 550 nm, and 41.7% at 800 nm, respectively.