An autoradiographic technique in estimating size of air-borne plutonium was studied by using ZnS(Ag) intensifier screen and Polaroid film. Effects of screen thickness, a weight percent of binder added and a particle size of ZnS(Ag) powder on a spot size of autoradiograph were examined. A calibration curve between alfa activity and spot size of autoradiograph produced under optimum exposure condition is presented. All procedures of the autoradiography can be performed without darkroom by the use of an improved Polaroid camera which has a double light-tight attachment.