Abstract: The emulsion droplets generation process in co-flowing microfluidic channels was studied by using a volume-of-fluid (VOF) numerical way. The diameter of emulsion droplets was controlled by the parameters including velocity ratio, surface tension and viscosity. Simulation results demonstrated that the diameter could be tuned by logarithm of velocity ratio in linear ways. The diameter increases with the increase of surface tension but becomes steady when surface tension is above 0.03 N/m. And the viscosity of disperse phase has negligible impact on droplet diameter. Furthermore, divinyl benzene (DVB) emulsion droplets with a diameter of 0.5 mm to 3.0 mm were produced in a self-made polydimethylsiloxane (PDMS) microfluidic slab where polyvinyl alcohol (PVA), DVB solution were set as the continuous phase and disperse phase respectively.