A portable transistorised fl-ray gauge of transmission type for the determination of thickness of paper has been developed and the results are reported in this paper. This instrument will be found useful in paper industries for quality control. The electronic circuitry has been found to be linear over the range 0.05-0.70 mg/cm with an accuracy better than 1.5% upto about 0.5 mg/Cm and within 2.9% above 0.5 mg/Cm .