A series of stiffened cylinders with R/t ratios of 300, 562, and 1070 were tested for value of Z = [b²/(R t)] 1-ν² between 70 and 500. In order to determine the stiffening factor, a series of unstiffened cylinders with the same R/t ratios as these stiffened were tested. The buckling stress coefficients of the skin Ks between the stiffeners were determined in terms of the size and the spacing of the stiffeners. The best-fit curves for the buckling stress coefficient and the stiffening factor were obtained by using the regression analysis and the IBM 1620 computer. The results of this experiment can be used directly in designing a longitudinally stiffened cylinder subjected to axial compression. However, it is incumbent on the designer to decide what metal distribution between the skin and the stiffeners would be selected so that the design would be the most economical one.