The origination of magnetism in Co-doped ZnO-based dilute magnetic semiconductor (DMS) with room temperature ferromagnetism (RTFM) has some controversy. In this paper, Zn 1-x Co x O nanoparticles (x = 0.00, 0.01, 0.03 and 0.05) were prepared using sol-gel method. The relationship between oxygen vacancy or carrier concentration and magnetic properties were investigated. The hexagonal wurtzite structure of all the samples was confirmed by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The morphological features were investigated by scanning electron microscopy (SEM) and transmission electron microscope (TEM), and the optical properties, including energy gap, were analyzed by ultraviolet–visible spectroscopy (UV–vis). The number of oxygen vacancies (V O) was measured by X-ray photoelectron spectroscopy (XPS), which is positively correlated with the Co content. The RTFM properties were analyzed by physical properties measurement system (PPMS). The results reveal that a higher carrier concentration is critical to promoting ferromagnetism. And, the magnetism of Co-doped ZnO may originate from V O defects or the carrier. Dear reviewer, We submit our manuscript entitled "Optimizing physical properties of Co-doped ZnO nanoparticles: controlling oxygen vacancy and carrier concentration" to Vacuum, and there are three highlights in our manuscript as following. 1. The diamagnetism of ZnO can be effectively transformed into ferromagnetism when ZnO is doped at a lower Co concentration. 2. The saturation magnetization (Ms) is larger than the reported value. Moreover, there is little difference in remnant magnetization (Mr). The coercivity (Hc) is lower than that of literature, and decreases as the Co content increases. 3. When Co content is 1 %, the RTFM is mainly induced by Vo defects. With the further increase of Co content, the RTFM is mainly induced by Carrier concentration, and it shows a monotonous increasing trend. [ABSTRACT FROM AUTHOR]