An analysis of asymmetric etch demonstrated that the nuclear track membranes can be prepared with smaller pore size than that with the normal etch. The experiment showed that the pore size can be decreased to 1/2 of the normal size by this method. The analysis for an etch method with temperature gradient showed that the pore size decreased with the increasing temperature gradient and could decrease to one-fifth of the normal size under the practical conditions. The computer simulation showed that the nuclear track membranes could prepared with cylinder pore by this method but it was difficult to control the etching process.