Functionally Graded Materials (FGMs) are a class of composites that have continuous variation of material properties from one surface to another. So, unlike composite laminates, there is no stress concentration. FGMs offer great promise in applications where the operating conditions are severe, including spacecraft heat shields, heat exchanger tubes, plasma facings for fusion reactors and engine components. In the present study, sound transmission through FGM cylindrical thin shell is investigated. The shell is assumed to be infinitely long and subjected to harmonic plane waves. The shell is immersed in a fluid medium and an external fluid (air) passes over it with a uniform speed. An exact solution is obtained by solving the shell equations in three directions and acoustic wave equations simultaneously. The transmission losses (TLs) obtained from the numerical solution are compared with those of other authors. Comparison shows a good agreement between the results. Because the pressure and displacement terms are expressed in infinite series form, the convergence checking is performed. Numerical results are used to show the effects of structural properties and flight conditions such as Mach number, flight level and volume fraction. The results shows as volume fraction exponent increases the TL decreasesin stiffness region, but the TL increases at in frequency higher than the ring frequency (mass-controlled region).