Separation of ultrafine particles by inertial effect in a supersonic flow field has been experimentally studied. The experiment was carried out in a Laval nozzle with different configurations. As a result, the cut size was found to decrease as Mach number increases, i.e., gas velocity becomes faster and the smallest cut size by this experiment has an aerodynamic diameter of about 0.04 mum at Mach number of about 2.8. which is about one order of magnitude smaller than that of conventional impactor. It is also much smaller than the performance of so-called low-pressure impactor and a fairly clear difference was observed among the configuration of nozzle and clearance between nozzle outlet to collection plate. Separation efficiency of the present impactor was evaluated by a Stokes number root Stk and was found to increase very steeply against root Stk, which is much steeper than the obtained separation curve for conventional impactor operating under atmospheric pressure.