An aerosolization technique has been developed to measure liquid-borne nanoparticles down to 30 nm and applied to evaluate retention efficiencies of liquid filters. This technique involves dispersing nanoparticle suspensions into air-borne form and measuring the size and concentration by a differential mobility analyzer coupled to a condensation particle counter. Polystyrene latex particles larger and smaller than 70 nm in diameter were dispersed by a constant output atomizer, COA, and an electrospray aerosol generator, ES, respectively, to avoid the interference from residue particles. With the ES, residue particles can be controlled to be less than 10 nm, allowing latex particles as small as 30 nm to be clearly distinguished from the size distribution measurements. Calibrations with 30, 50, 125, and 200 nm latex particles showed that liquid-borne and air-borne particle concentrations are proportionally related. This provides an effective way to quantify liquid-borne particles as small as 30 nm, which cannot be analyzed by state-of-the-art liquid particle counters. An application of this technique is to evaluate the nanoparticle retention performance of liquid filters. Both 200 and 400 nm rated Nuclepore filters were challenged with latex particles of different sizes, and retention efficiency as a function of particle size was determined by comparing the particle concentrations upstream and downstream of the tested filters. The results are comparable with the nominal pore size stated by the manufacturer if sieving is the dominant filtration mechanism and demonstrate the feasibility of using the aerosolization technique to evaluate the retention efficiency of filters against nanoparticles in liquids.