The free vibration of cylindrical thin shells made of functionally graded materials was studied. The gradient properties of functionally graded material were expressed as volume fraction power-law distributions in the thickness direction. Based on Donnell’s shell theory, the simplified governing equations for linear vibration of functionally graded cylindrical thin shells were derived. Free vibration of functionally graded cylindrical thin shells was analyzed with the present shell theory. Frequency formula for simply supported functionally graded cylindrical thin shells was obtained. By comparing the results for simply supported cylindrical shells with both previous works and finite element method, the present simplified functionally graded cylindrical thin shell theory was validated. The influence of circumference wave number and power-low exponential on frequencies was also discussed.