Abstract: To study the size distribution of microbubbles generated from the multiphase pump dissolved air flotation process,the laser diffraction technology was used.The influence of pressure,salinity,coagulant,frother and oil content on bubble size was investigated.Resultsshowed that increasing air pressure had a positive effect on bubble size;higher salinity could decrease the bubble size distribution but no effect when it was higher than 0.5%;polyaluminum chloride(PAC) less than the critical micelle concentration signiflcantly inhibited bubble coalescence,and higher concentrations of the reagents could reduce the mean bubble diameter by 36%;frother could steady bubbles,but no effect on bubble size improvement;and oily water with small surface tension reduced the bubble size.