In order to consider ability of absorbing impact energy in functionally graded materials (FGM), longitudinal impact problem of a FGM bar was analyzed mathematically by using Laplace transformation and numerically inverse transformation. The FGM bar was subjected to step impact load at one end, and was fixed at the other end. The longitudinal elastic modulus in the FGM bar was assumed to be proportional to the square of its density. As the numerical results, in the FGM bar having increasing longitudinal modulus from the impact end to the fixed end, the larger compressive stress and even large tensile stress occurred near the fixed end compared with the ones in uniform bar. The FGM bar was clarified to have ability of high impact energy absorption by collapse and/or fracture. In the FGM bar having decreasing modulus from the impact end, the compressive stress was approximately the same as the one in the uniform bar, and the history of the stress varied smoothly. The impact energy absorption in the bar was predicted to be damping.