Microbubble size distribution is an important parameter of DAF in water and wastewater treatment. In this study, a new image method with a sampling tube was developed to measure the microbubble size in the DAF process. In addition, the effects of saturator pressure, TX-100 concentration, and polyDADMAC concentration in the saturator on bubble size distribution were investigated. The results showed that the bubble size decreased with the increase of the saturator pressure when the pressure was below 0.4 MPa and the change of bubble size was insignificant when the pressure increased further. The bubble size decreased with the increase of TX-100 concentration, while polyDADMAC had a minimal effect on the bubble size in the range of 0-5 ppm. This study indicated that the method was a rapid and robust one, which may be applied in the industrial-scale DAF.